FROM BEDROCK MONITORING

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1 HYDROGEOLOGIC OBSERVATIONS FROM BEDROCK MONITORING WELL NESTS in the BIG SPRING BASIN Open-File Reprt 90-1 Pslviihe Luana aa Iwa Department f Natural Resurces Larry J. Wilsn, Directr December 1990

2 HYDROGEOLOGIC OBSERVATIONS FROM BEDROCK MONITORING WELL NESTS in the BIG SPRING BASIN Open-File Reprt 90-1 A Reprt f The Big Spring Basin Demnstratin Prject Prepared by Rbert D. Rwden and Rbert D. Libra Energy and Gelgical Resurces Divisin Gelgical Survey Bureau The BigSpring Basin Demnstratin Prject f the Iwa Department f Natural Resurces is supprted, in part, thrugh the IwaGrundwater Prtectin Act and Petrleum Vilatin Escrw accunts, and ther spnsringagencies: The U.S. Department f Agriculture, SilCnservatin Service, the U.S. Envirnmental Prtectin Agency, Regin VII, Kansas City, and the Nnpint Surce Prgrams Office, Washingtn, D.C, the Iwa State University Cperative Extensin Service, and the U.S. Gelgical Survey, Water Resurces Divisin. December 1990 Iwa Department f Natural Resurces Larry J. Wilsn, Directr

3 TABLE OF CONTENTS Page ABSTRACT 1 INTRODUCTION 3 Gelgic setting 3 Grundwater Flw 3 Mnitring Wells 5 Stratigraphic Framewrk 5 The Galena Aquifer 5 Water Table/Ptentimetric Surface in the Galena Aquifer 7 Physigraphy and Grundwater in the Basin 7 Majr Cnduit Znes and Grundwater Flw in the Galena Aquifer 10 KARST FEATURES, GALENA STRUCTURE, AND LITHOLOGY 10 Hydrgelgic Crss-Sectins 11 HYDROSTRATIGRAPHY OF MONITORING-WELL SITES 11 Site BS-1 11 SiteBS-2 21 SiteBS-3 21 SiteBS-4 22 Site BS-5 23 WELL MONITORING INSTALLATIONS 23 PRELIMINARY WATER-LEVEL DATA (January 1989 t September 1989) 23 PLANNED WATER-QUALITY AND WATER-LEVEL DATA 24 ACKNOWLEDGEMENTS 25 REFERENCES 27 in

4 LIST OF FIGURES Page Figure 1. Figure 2. Figure 3. Figure 4. Figure5. Figure 6. Bedrck gelgic map f the Big Spring study area shwing mnitring-well sites (adapted frm Hallberg etal., 1983) 4 Generalized stratigraphic sectin fr BigSpring study area (frm Hallberg etal., 1983) 6 Elevatin f the water table/ptentimetric surface in the Galena aquifer, and lines f crss-sectin shwn n figures 5 thrugh 7 (adapted frm Hallberg etal., 1983) 8 Grundwater basin and subbasin divides, and schematic grundwater flw lines based n the water table/ptentimetric surface map (adapted frm Hallberg etal., 1983) 9 Hydrgelgic crss-sectin A-B; lcatin shwn n figure 3. Als shwn are prjected lcatins f mnitring-well sites (adapted frm Hallberg etal., 1983). See figure 1 fr legend 12 Hydrgelgic crss-sectin C-D; lcatin shwn n figure 3. Als shwn are prjected lcatins f mnitring-well sites (adapted frm Hallberg etal., 1983). See figure 1 fr legend 12 Figure 7. Hydrgelgic crss-sectin E-F-G; lcatin shwn n figure 3. Als shwn are prjected lcatins f mnitring-well sites (adapted frm Hallberg etal., 1983). See figure 1 fr legend 13 Figure 8. Lithlgic key fr figures 9 thrugh Figure 9. Stratigraphic sequence and mnitring-well installatins at site BS-1 (cuttings lgged by M. J. Bunk) 15 Figure 10. Stratigraphic sequence and mnitring-well installatins at site BS-2 (cre lgged by M. J. Bunk) 16 Figure 11. Gelgic crss-sectin and mnitring-well installatins at site BS-2. Shaded areas represent dcumented r inferred slutin vids and slutinally enhanced fractures and bedding planes (cre lgged by M. J. Bunk) 17 Figure 12. Stratigraphic sequence and mnitring-well installatins at site BS-3 (cre lgged by M. J. Bunk) 18 Figure 13. Stratigraphic sequence and mnitring-well installatins at site BS-4 (cre lgged by M. J. Bunk) 19 LIST OF TABLES Table 1. Page Ptentimetric elevatins frm mnitring wells at sites BS-1 thrugh BS-4. Als shwn are frmatins screened and depth f screened interval belw land surface 20

5 ABSTRACT The Big Spring basin isa 103 mi2 (267 km2) grundwater basin in Claytn Cunty, Iwa. The grundwater discharge and water quality within the basin have been mni tred cntinuusly since Big Spring discharges frm the Galena aquifer, the main grundwater surce in the basin. It is a carbnate aquifer with mderate karst develp ment. In previus studies, water table/ptentimetric data frm Galena wells and spring elevatins, alng with dye tracing and identificatin f lsing- and gainingstream reaches, were used t define the extent f the grundwater basin. T refine the three-dimensinal understanding f the grundwater-flw system, test hles were drilled at five sites. At fur f these sites, cres were taken and nested mnitring wells were installed. Well sites were placed in different regins f the basin t delineate the hydrlgic system, and t refine the ptentimetric relatins within the flw system f the Galena aquifer and bunding aquitards. Site BS-1 is lcated in the suthernmst prtin f the basin near the Big Spring and the Turkey River. In this prtin f the basin, the tp f the Galena ptentimetric surface declines t within 50 t 75 feet (15 t 23 m) f the base f the aquifer. The Turkey River acts as "base-level" fr the Galena ptentimetric surface. The bedrck dips tward this area and grundwater is discharged thrugh Big Spring and assciated smaller springs t the Turkey River. At this lcatin, hydraulic head increases with depth, and the ptential fr grundwater mvement is upward. Site BS-2 is lcated near the center f the basin, abve an inferred majr cnduit zne. In this prtin f the basin, Rbert's Creek and Silver Creek and their assciated alluvial valleys are 100 feet (31 m) r mre abve the Galena ptentimetric surface, which lies rughly in the middle f the aquifer. In this prtin f the basin the ptential fr grundwater mvement is dwnward. A cmplex system f small vids was encuntered at this site. Site BS-3 is lcated in the nrth-central prtin f the basin near an area f sinkhle develpment. At this lcatin, the Galena ptentimetric surface was expected t be near r abve the tp f the Galena aquifer. Installatin f the lwer Galena bservatin well shwed the ptentimetric surface t be apprximately 125 feet (38 m) belw the tp f the Galena at this lcatin. This site may be ver a cnduit zne where the Galena ptentimetric surface is lcally depressed. The feature may nt be large enugh t have been reflected in previus assessments f water-level data. At this lcatin, the ptential fr grundwater mvement is als dwnward frm the Galena t the St. Peter. Site BS-4 is lcated in the nrthwest prtin f the basin. In this area, Galena carbnates are verlain by the Maquketa Frmatin, and the Galena ptentimetric surface is lcated abve the tp f Galena. At this site, an anmalusly thick sequence f Quaternary materials verlie an incmplete Maquketa sectin. In this prtin f the basin, the ptential fr grundwater mvement is dwnward; the Quaternary and Maquketa rcks frm an aquitard, and the Galena is a cnfined aquifer. Fr cntinuus mnitring f the wells, digital recrders were installed and set t register water levels hurly. Frm January 1989 t September 1989, all mnitring wells exhibited verall declines f mean mnthly water levels, reflecting the cntinued absence f recharge during the drught. The mre shallw wells exhibited mre immediate respnse t snw melt and rain fall events than the deeper mnitring wells. Water-quality sampling f the mnitring wells is expected t begin during the 1990 water year. Onging mnitring f water levels in wells will prvide detailed three-dimensinal bservatins f changes in the varius ptentimetric surfaces within the basin, and refine knwledge f ptentimetric relatins and cntaminant transprt within the flw system f the Galena aquifer and bunding aquitards.

6 INTRODUCTION Since 1980, the Department f Natural Resurces, Gelgical Survey Bureau (GSB), in cnjunctin with numerus state, federal, and lcal agencies, and university researchers, have been investigating the impact f agricultural chemicals, particularly nitrgen fertilizers and pesticides, n grundwater. Investigatins by Hallberg and Hyer, 1982; Hallberg et al., 1983, 1984a, 1984b, 1985, 1986, 1989; and Libra et al., 1984 have dcumented the magnitude f grundwater cntaminatin related t agricultural practices, identified hydrgelgic settings that are susceptible t cntaminatin frm agricultural use, and prvided insights int the mechanisms that deliver agricultural chemicals t grundwater. A significant prtin f this wrk has fcused n the Big Spring basin, a 103 square mile (267 sq. km) grundwater basin lcated in Claytn Cunty, in nrtheast Iwa (Figure 1). T further refine the three-dimensinal understanding f the grundwater flw system in the basin, a series f nested mnitring wells were installed in 1988 and This reprt will summarize the hydrlgic placement f the mnitring wells, the infrmatin btained frm the cre analysis and will als present initial mnitring results frm the wells. Gelgic Setting The basin is lcated within the Palezic Plateau Landfrm regin in nrtheast Iwa (Hallberg et al., 1984b). Tpgraphically, the basin varies frm mderately rlling in the nrthern ne-half f the area, t steeply slping near the Turkey River Valley in the suthern prtin f the area. Ttal relief in the basin is apprximately 420 feet (128 m), with as much as 320 feet (98 m) f relief ccurring alng the Turkey River Valley in the suthwest crner f the basin (Hallberg et al., 1983). Bedrck units in the basin include Silurian and Ordvician strata. The Galena aquifer is the main grundwater surce in the basin; Big Spring functins as the main grundwater discharge pint fr this carbnate aquifer. The gegraphic extent f the Big Spring grundwater basin was delineated in previus investigatins thrugh defining the water table/ptentimetric surface f the Galena aquifer, dye traces, and lcating and gaging gaining- and lsing-stream reaches. Land use within the basin is essentially all agricultural. There are n majr urban r industrial areas, n landfills, cmmercial feedlts, r ther majr pint surces t significantly affect grundwater quality. The nly pint surces within the basin are surface-water discharges frm a creamery and a new sewage treatment plant. The effects f these discharges are mnitred. As f 1987, abut 43% f the basin was in rw crp, abut 14% was strip crpped, abut 35% in cver crps, and abut 5% f the basin is frest. The remaining prtin f the basin is cmprised f small urban areas, quarries, and rads. Up until abut 1986, 99% f the rw crp acreage was in crn. Since then, there have been small increases in the amunt f sybeans and srghum grwn in the area. The cver crps grwn within the basin include haycrps, ats, and pasture, and recently small amunts f wheat. Land use is interpreted frm aerial phts, ASCS recrds, staff field ntes, and land wner surveys. Grundwater Flw The grundwater discharge and water quality at Big Spring, have been mnitred cntinuusly since Nvember 1981 (Hallberg et al., 1983). Nearly 90% f the grundwater discharged frm the basin t the Turkey River ccurs thrugh Big Spring, with the remainder discharged via a few small assciated springs. Grundwater als discharges dwnward frm the Galena aquifer t underlying units. The water frm Big Spring supplies a Department f Natural Resurces, state-perated trut hatchery. The discharge is cntrlled by a cncrete dam, which abuts bedrck surrunding the spring. At lw stages, all water is discharged thrugh a 30 inch (0.76 m) diameter, crrugated-metal pipe int the hatchery system. At higher stages, water discharges bth thrugh the pipe and thrugh a cncrete spillway t the Turkey River. These structures allw the discharge t be gaged. The Galena is a carbnate aquifer with mderate karst develpment. Where the Galena

7 SILURIAN n ORDOVICIAN BEDROCK GEOLOGIC MAP Big Spring Study Are Apprximate lcatin f sinkhles 1982 Su-Silurian Okxniies (BtntJing, Tete des Mm*. M*iem Frm* ) Cmpiled by G A Ludvigsn (field mpping by G A Ludvigsn, R.M McKay, MJ Bunk, S.J Lenker) N N 3 mi km. Ap*«iml«lcatin f sm*h*e» a-gien crbna'es 5utxx)ue. Wi«e Lake, nd Dunltith Fr Ovp 0*p - St Peiw Snd»tn* Omf - Mquketa Frm Ft Atkinsn, Clermnt, nd Elgin MemDers Odp OdD-D«c<h, PtileviJK, nd Genwd Fn Ovti I OUi- Slkp«e F.m Figure 1. Bedrck gelgic map f the Big Spring study area shwing mnitring-well sites (adapted frm Hallberg et al., 1983).

8 carbnates lie clse t the land surface, slutinal features such as sinkhles and enlarged fractures are present t varying degrees. The grundwater flw system within the Galena cnsists f bth cnduit- and diffuse-flw cmpnents. The cnduit cmpnent is recharged by intermittent diversin f surface-water runff int sinkhles; this recharge typically accunts fr abut 10% f the ttal grundwater recharge t the system. The diffuse-flw system cntributes abut 90% f the recharge, via infiltratin thrugh the sil and rck materials underlying the surface. The tw systems are cmplexly related. Mnitring Wells The current test drilling was undertaken t further delineate the hydrgelgic system within the Big Spring basin, t help place the Galena flw system in a three-dimensinal perspective, and t refine the knwledge f ptentimetric relatins within the flw system f the Galena aquifer and surrunding aquitards. T facilitate this, at five sites, (Figure 1) hles penetrating the Galena aquifer were drilled between June 1988 and July At fur sites, cres were taken frm varius intervals and mnitring wells were installed. The cring allwed fr a mre detailed hydrstratigraphic analysis f the basin. Placement f mnitring-weli nests at each site will assist further dcumenting the three-dimensinal distributin f ptentimetric elevatins, and therefre the lateral and vertical cmpnents f water flux within the basin. The wells als allw fr water-quality sampling within discrete stratigraphic intervals. Figure 1 shws the lcatin f the mnitring-well sites with respect t the bedrck gelgy in the basin area. In discussing the infrmatin btained frm the installatin f the wells, the framewrk f the Big Spring basin hydrgelgy is briefly summarized (Hallberg et al., 1983). Stratigraphic Framewrk The ldest rcks expsed in the Big Spring basin area are the carbnate rcks f the Shakpee Frmatin f Ordvician age (Figures 1 and 2). The Shakpee is uncnfrmably verlain by the St. Peter Sandstne, which is variable in thickness and frms an aquifer f lcal imprtance in nrtheast Iwa. Overlying the St. Peter are the shales, shaly carbnates and carbnates f the Glenwd, Platteville and Decrah frmatins. The Decrah, Platteville and Glenwd frmatins frm an aquitard which separates the St. Peter aquifer frm the Galena aquifer. Overlying the Decrah Frmatin are the Dunleith, Wise Lake, and Dubuque frmatins. The Galena Grup includes the Decrah, Dunleith, Wise Lake, and Dubuque frmatins. Overlying the Galena Grup are the rcks f the Maquketa Frmatin which are, in ascending rder, the Elgin Member, the Clermnt Shale Member, the Ft. Atkinsn Member, and the Brainard Shale Member. The uppermst member f the Maquketa Frmatin, the Brainard Shale, frms a majr aquitard in eastern Iwa, whereas the lwest member, the Elgin, is in part hydrlgically cnnected with the Galena carbnates. Overlying the Maquketa Frmatin are Silurian dlmites, which are the yungest bedrck units in the study area. In the Big Spring basin, the Silurian ccurs as scattered ersinal remnants, but frms an imprtant reginal aquifer t the suth and west. The Galena Aquifer The Galena aquifer is made up f three frma tins f the Galena Grup: the Dunleith, Wise Lake, and Dubuque frmatins. The lwest fr matin in the Galena grup, the Decrah, is part f the aquitard separating the Galena aquifer frm the St. Peter Sandstne. The Galena aquifer is cmprised f interbedded limestnes and dlmites, with sme shaly interbeds. The shaly interbeds ccur primarily in the Dubuque Frmatin. Reginally, the degree f dlmitizatin decreases t the nrth. The Dubuque Fr matin tends t be well-bedded, with shaly part ings, whereas the Wise Lake is mre massive, and the Dunleith tends t be cherty. Within the study area, the Galena aquifer is apprximately 220 feet (67 m) thick. Structurally, the aquifer exhibits a flexure running rughly nrth-suth thrugh the central prtin f the basin and dip ping t the suthwest at abut 18 feet/mile

9 nwltn Nte' Silurian thins t nrth beneath the Devnian. Devnian rests directly n the Maquketa Fm. in Winneshiek Cunty. Silurin limestne (Wucma Fm.) utcrp in nrthern Fayette Cunty Primarily dlmite elsewhere. IOOi MM 300- CRETACEOUS PERIOD Windrw Frmatin (lcally present abve Galena carbnates) limestne /, dlmite / shale sandstne l brecciated limestne 1- sandy limestne / sandy dlmite z=- argillaceus r shaly ^fzi siltstnes r silty *s chert ppp phsphatic irn xides. */. gravel vvww majr ersinal uncnfrmity ldest bedrck expsed in nrtheast Iwa Figure 2. Generalized stratigraphic sectin fr Big Spring study area (frm Hallberg et al., 1983).

10 (3.5 m/km) (Hallberg et al., 1983). All units within the Galena aquifer are jinted r fractured. At nearly any expsure in the study area, sme slutinal activity alng jints and bedding planes is apparent. Majr jints have been widened by slutinal activity and cntain secndary depsits f calcium carbnate. Within the Big Spring basin, cncentratins f sinkhles are fund near the utcrp area f the cntact between the Galena Grup and lwer Maquketa Frmatin (Figure 1). Sinkhle develpment is mre prminent in regins where the Galena carbnates have a brad utcrp area and in lcalities where less than 20 feet (6.1 m) f the lwermst Maquketa Frmatin verlies the Galena. Newly frmed sinkhles investigated by GSB staff have revealed pen, vertical slutin shafts penetrating up t 120 feet (37 m) belw the land surface, int the aquifer. In areas where the tpgraphy is mre steep, fewer sinkhles are apparent. Water Table/Ptentimetric Surface in the Galena Aquifer In previus hydrgelgic studies, a water table/ptentimetric surface map fr the Galena aquifer was cmpiled (Figure 3) frm water levels in Galena welis and measurements f Galena spring elevatins (Hallberg et al., 1983). The cnturing, and definitin f the basin was ass isted by dye-trace studies and defining lsingand gaining-stream reaches. The ptentimetric cnturs define the grundwater basin divide and indicate the general directin f grundwater flw. Interpreted grundwater flw ccurs n a reginal scale at right angles t the ptentimet ric cnturs. Within the Big Spring basin, the Galena aquifer exhibits cnfined and uncnfined aquifer cnditins. In the western part f the basin, where a thick sequence f verlying Maquketa Frmatin is present, the Galena acts as a cnfined aquifer. Within the rest f the basin the Galena is uncnfined. Physigraphy and Grundwater in the Basin The Big Spring basin includes an area f apprximately 103 square miles (267 sq. km) and cntains mst f the surface-drainage basin f Rbert's Creek. On the nrth and west, the grundwater-basin divide (Figure 4) is nearly cincident with the surface-drainage divide, including the Rbert's Creek system and an unnamed creek which empties int the Turkey River near Big Spring. On the east side f the basin, the grundwater divide cuts acrss the surface-drainage basins f Bldy Run, Hward and Rbert's Creeks and tributaries. Grundwater flws frm the divide tward Big Spring, and discharges frm the Galena aquifer thrugh a narrw regin t theturkey River, with the majr prtin cncentrated at Big Spring, Back Spring and Heick's Spring. Past wrk has shwn that Big Spring cnstitutes abut 89% f the grundwater discharge. The relatinship between the surface-water system and the grundwater system within the Big Spring basin is very cmplex. In their headwaters, mst f the streams are recharged by shallw-grundwater flw frm lcal seeps, r diffuse grundwater flw frm the Maquketa, Galena, and/r Quaternary depsits. In the central and eastern prtins f the basin, the streams and theiralluvial valleys are perched well abve the Galena ptentimetric surface. Here streams lse surface water t the grundwater system thrugh intermittent runff int sinkhles, and as diffuse seepage in sme perennial streams. As the streams leave the basin, in the St. Olaf area, they receive discharge frm the Galena aquifer, and again becme gaining streams. Mst streams maintain perennial flws thrugh prtins f the basin that appear t be lsing reaches. This is pssible when sustained recharge, prvided by shallw grundwater (including tile drainage) in the streams headwaters, is greater than the rate f leakage int the grundwater system dwnstream. The upper prtins f the alluvial depsits and large areas f the stream beds are relatively fine-textured, silty depsits, which prvide relatively slw perclatin, effectively retarding lsses r leakage thrugh the stream bed. Much f the well-integrated, dendritic drainage netwrk develped in the Big Spring basin is cntrlled by bedrck, especially the secnd rder and larger valleys in the eastern tw-thirds f the basin. Many valleys in this area appear t fllw majr jint trends. Several small

11 R6WIR5W Crss-sectin Lines N 3 mi. / ~/ Ptentimentric Cnturs cntur interval = 50 feet 0 I 2 3 4km. Grund-water Basin Divide Figure 3. Elevatin f the water table/ptentimetric surface in the Galena aquifer, and lines f crsssectin shwn n figures 5 thrugh 7 (adapted frm Hallberg et al., 1983).

12 R6WIR5W _AIJamakee C. Claytn C. Grund-water Basin Divide N I! 3 mi. Grund-water Sub-basin Divides km. Grund-water Flw Lines Figure 4. Grundwater basin and subbasin divides, and schematic grundwater flw lines based n the water table/ptentimetric surface map (adapted frm Hallberg et al., 1983).

13 blind valleys in this area disrupt the integrated drainage netwrk and lead t the develpment f hllws which discharge entirely int sinkhles, entering the grundwater system f the Galena aquifer. Hwever, infiltratin rates thrugh the sil and rck are relatively high in this area and runff is infrequent. The sinkhle basins ccupy 11.5 square miles (29.8 sq. km), r abut 11% f the grundwater basin. Majr Cnduit Znes and Grundwater Flw in the Galena Aquifer The wide range f permeabilities that ccur in carbnate aquifers result in varying types f grundwater flw and recharge/discharge mechanisms. The terms cnduit flw and diffuse flw were used by White (1977) t describe tw types f flw that ccur in carbnate aquifers. Cnduit flw describes grundwater mvement thrugh larger pen cavernus znes r cnduits. The term diffuse flw characterizes flw thrugh relatively unmdified fractures and bedding planes. Within the Galena aquifer, bth diffuse- and cnduit-flw systems are present. The high transmissivity f the cnduit znes allws fr rapid grundwater flw, which draws dwn the water table/ptentimetric surface within the adjacent diffuse znes. This enhances flw frm the diffuse-flw system tward the cnduit znes. The cnfiguratin f the water table/ ptentimetric surface (Figure 3) prvides an indicatin f the lcatin f majr cnduit znes in the Galena aquifer. Grundwater flws frm high t lw ptentimetric elevatins, and n a reginal scale, flws at right angles t the pten timetric cnturs. Figure 4 is a schematic flw diagram fr the Galena aquifer based n the water table/ptentimetric surface. The flw lines cnverge twards prnunced trughs, r lws in the ptentimetric cnturs in the central and eastern sub-basins. These trughs cn verge and flw tward the grundwater dis charge area alng the Turkey River, principally at Big Spring. These elngate trughs have rela tively high transmissivities, relative t adjacent parts f the aquifer and are interpreted as majr "arterial" rutes f the cnduit-flw system, which transmit grundwater frm sinkhles t Big Spring. These trughs likely include a brader zne f slutinally enhanced fracture permeab ility. In the western sub-basins, the thick cap f Maquketa Frmatin and resulting lack f sinkhles in the area limit the ptential fr develpment f pen slutin cnduits. In areas farther away frm the interpreted cnduit-flw system, the water table/ptentimetric cnturs and grundwater flw lines are mre regular in shape and distributin, suggesting that in these areas, the Galena acts as a diffuse-flw aquifer. Recharge t the diffuse-flw system is by infiltratin thrugh the sil and rck units. Infiltrating recharge water may fllw a cmplex path thrugh vertical jints and fractures befre reaching the zne f saturatin within the Galena aquifer. Lw permeability units such as glacial tills and shales f the Maquketa Frmatin, which verlay the Galena aquifer, retard dwnward infiltratin and create shallw grundwater flw systems abve the Galena. Flw within this shallw grundwater system is cntrlled by lcal tpgraphy and the distributin and thickness f lw permeability units. In the nrth and nrthwest part f the basin, this tpgraphic cntrl is evidenced by the presence f small springs r seeps alng majr drainages, such as the headwaters f Rbert's and Silver Creeks. KARST FEATURES, GALENA STRUCTURE, AND LITHOLOGY The distributin f karst-slutinal features within carbnate rcks is cntrlled by the structural, and lithlgic prperties f the rcks in relatin t grundwater-flw directins (Thrailkill, 1968; Pwell, 1977; White, 1977; Hallberg and Hyer, 1982; and Bunk, 1983). Structural features such as jints, fractures, bedding planes, faults, flexures, and high fracture densities ften prvide areas where intense slutinal activity may be lcalized. Fr example, the central cnduit zne in the Big Spring basin (Figure 4) appears cincident with the nrth-suth trending flexure in the Galena aquifer previusly nted. Other cnduit-zne trughs are als cincident with majr stream-valley systems, even thugh the 10

14 ptentimetric surface is deep in the subsurface. The cincidence f the cnduit znes, r "ptentimetric valleys", and surface tpgraphy suggests that structural features f the Galena and Maquketa rcks have influenced the lcatin and develpment f the stream valleys and prminent slutin-cnduit znes within the Big Spring basin. The lithlgy f the Galena carbnates prbably als plays a rle in the distributin f karst features. Limestne is mre sluble than dlmite, and is mre susceptible t the develpment f karst-slutin features. In the Big Spring basin, the Dubuque and Dunleith frmatins are dminantly limestne, whereas the Wise Lake Frmatin is mre dlmitic and massive. Hrizntal slutinal features, are better develped in the Dubuque and Dunleith frmatins. In the Wise Lake Frmatin, slutinal features are generally enlarged vertical jints, fractures, and ccasinally dme pits. In the Big Spring study area, the majr sinkhle cncentratins ccur where the tp f the Galena (the Dubuque Frmatin) utcrps in the nrth-central prtin f the basin. Further suth, where the Dubuque has been largely remved by ersin and disslutin and the Wise Lake utcrps, there are fewer sinkhles. Hydrgelgic Crss-Sectins Hydrgelgic crss-sectins f the Big Spring basin have been cnstructed t help place the Galena flw system in a three-dimensinal perspective (Hallberg et al., 1983). The crss-sectin lines are lcated n Figure 3, and shwn n Figures 5, 6, and 7. The crss-sectins delineate the gelgic units, the general tpgraphy and structure, streams, relatins t the St. Peter Sandstne (Osp), and the majr sinkhle areas. Als shwn are the apprximate lcatins f sites BS-1 thrugh BS-5. Crss-sectin A-B (Figure 5) runs acrss the nrthern grundwater and surface-water divide, and then rughly parallels a grundwater-flw path int the discharge area at Big Spring. This sectin als parallels the structural dip f the Galena aquifer. Alng this crss-sectin, the water table/ptentimetric surface declines in elevatin near the central cnduit-zne trugh; Silver Creek and Rbert's Creek, and their assciated alluvial valleys are 100 feet (31 m) r mre abve the Galena ptentimetric surface in sme areas. In this area, streams may lse water int the grundwater system. Crss-sectin C-D (Figure 6) runs east t west acrss the central and eastern cnduit-zne trughs in the Galena ptentimetric surface. In the central prtin f the sectin, Silver and Rbert's Creeks are again perched abve the Galena ptentimetric surface. In the western part f the sectin, the ptentimetric surface is essentially in cnfluence with Rbert's Creek and its assciated alluvial aquifer, prviding grundwater discharge t the streams. Crss-sectin E-F-G (Figure 7) runs east-west within the suthern part f the grundwater basin, crssing the ptentimetric surface (Figure 3) where the east cnduit-zne trugh trends t the west and suth t merge with the central trugh (Figure 4). T the west, this sectin runs thrugh utliers f Silurian rcks (Su) and areas verlain by rcks f the Maquketa Frmatin (Omb and Omf). In the central prtin f the sectin, Rbert's and Silver Creeks are lsing reaches. T the east, the sectin traverses Hward Creek and an unnamed tributary just nrth f where these creeks begin t frm gaining reaches again. HYDROSTRATIGRAPHY OF MONITORING-WELL SITES The analysis f the cres and cuttings frm five drilling sites has supprted the assessment f, and allwed refinement f the Big Spring basin hydrlgic system. At site BS-1, the gelgic lg f the stratigraphic sequence is based n drill cuttings; at sites BS-2 thrugh BS-4, the lgs f the stratigraphic sequences are based n cuttings and cres. The lithlgic key and stratigraphic sequences fr the fur sites are shwn in Figures 8 thrugh 13. Site BS-1 Site BS-1 is lcated in the suthernmst prtin f the basin (Figure 1) n the alluvial plain, within 300 feet (91 m) f Big Spring and the 11

15 GrwdaNr Figure 5. Hydrgelgic crss-sectin A-B; lcatin shwn n figure 3. Als shwn are prjected lca tins f mnitring-well sites (adapted frm Hallberg et al., 1983). See figure 1 fr legend. GrrOxMr DMdi El«v«ti*n in msten feat BS-4 Figure 6. Hydrgelgic crss-sectin C-D; lcatin shwn n figure 3. Als shwn are prjected lca tins f mnitring-well sites (adapted frm Hallberg et al., 1983). See figure 1 fr legend. 12

16 ItaMtta In mettrt '! OwM* Figure 7. Hydrgelgic crss-sectin E-F-G; lcatin shwn n figure 3. Als shwn are prjected lca tins f mnitring-well sites (adapted frm Hallberg et al., 1983). See figure 1 fr legend. Turkey River. The land surface at this site is abut 13 feet (4.0 m) abve the Turkey River at base flw, and abut 5 feet (1.5 m) abve the Big Spring pl at base flw. In this prtin f the basin, the tp f the Galena ptentimetric surface declines t within 50 t 75 feet (15 t 23 m) f the base f the aquifer (Figure 5), and the Turkey River acts as "base-level" fr the Galena ptentimetric surface. Well recrds and previus and nging studies f the alluvial histry f the Turkey River Valley have shwn that in the gelgic past, the Turkey River was dwncut 50 t 60 feet (15 t 18 m) deeper than the present fldplain (Hallberg et al., 1983). In this area, the river prbably cut t, r perhaps thrugh, the base f the Galena carbnates. This suggests that cnduits may have develped t the base f the aquifer, and that karst-cnduit flw paths may penetrate the full thickness f the Galena aquifer in parts f the basin. Figure 9 shws the stratigraphic sequence and mnitring-well installatins fr site BS-1. At this lcatin, the Dubuque, Wise Lake and much f the Dunleith frmatins have been truncated by dwncutting f the Turkey River. Fur mnitring wells, BS-1 A thrugh BS-1D were installed at varius depths abve, within and belw the Galena aquifer (Table 1). BS-1A is screened int uncnslidated sand, gravel and limestne bulders, which are verlying the Dunleith Frmatin. These depsits are thught t be a cmbinatin f alluvial stream depsits and clluvium frm dwncutting alng the valley wall. Water levels in this well are abut 13 t 15 feet (4.0 t 4.6 m) belw land surface. BS-1B is pen int the bttm f the Galena aquifer in the Dunleith Frmatin. In this well, the water level is abut 8 t 10 feet (2.4 t 3.1 m) belw land surface. The bservatin that the water level in BS-1A is abut 5 feet (1.5 m) lwer than the water level in BS-1B suggests that at this site, grundwater frm the lwer prtin f the Galena is discharging upward t the Turkey River and its alluvial aquifer. Mnitring well BS-1C is pen t the Pecatnlca Member f the Platteville Frmatin. 13

17 KEY r^ ^ jjn Argillaceus 0 Silty Clay Shaly Siltstner! Silt A A A Cherty Sand r Sandstne v w w Vuggy v v Sand and Gravel Shale, Calcitic Shale Dlmite / / Dlmitic Limestne r Calcitic Dlmite Limestne Figure 8. Lithlgic key fr figures 9 thrugh 13. U

18 en -n O t 5 3 a> t' c/> -Q O u =: a 3 CO i CO 3 01 CQ CO CL CT < a> St. Peter Sandstne CO I Q cn Platteville Pecatnica McGregr CO _ CO Decrah ^ Guttenberg n Dunleith CO f \3 O Quaternary und. CO : } -. 5 O 3 - sr "P CO -I m DO < s CO y N O 3 i CO c 3 c i I

19 Depth In Depth In meters 20 Oi - feet Fm CO Mbr. SITE BS-2 /////// ////// II II 1 I11 III II 1 III! 1 1*1 /./ / / / 1 / / / /./ /. 1 /. Nested Piezmeters elev. 945 ft. (288 m.) BS-2A7; Piezmeters 40 -C c Q N Sample Pssible Vid i A a A BS-2B12 BS-2C BS-2D 1*1 ' 1 * II Vid Rubble BS-2E & & Q C O i (3 v a V 4 * S.F ' ' > > 0_ G.S. ffl c75cl i 1 1! II.-'/// / / z=^^^^=^^^^==^== S-ak U 1 1 U 1 1 UI BS-2F / BS-2G z Figure 10. Stratigraphic sequence and mnitring-well installatins at site BS-2 (cre lgged by M. J. Bunk). 16

20 SITE BS-2 Piezmeters Depth In Depth In meters teet Fm_ Mbr Rberts c/> t ww w tn m m m m Creek : zfzi TtCL 1 i I VA 20 _ CIZ HZ / / / i z 100 : I, I J-L 5 r T i i i J L I I I I $m 1 T_r! I 1 S3 j^zn i, i ^ OS. fee. I'll.r- ^. S ' 100 Stvw 1 i > I I i I I I t t=l /,'/ // /.'!'/ I II I ', T feet metefs Figure 11. Gelgic crss-sectin and mnitring-well installatins at site BS-2. Shaded areas represent dcumented r inferred slutin vids and slutinally enhanced fractures and bedding planes (cre lgged by M. J. Bunk). When BS-1C was installed, drilling fluid was lst, near the tp f the Decrah Frmatin, suggesting a zne f high cnductivity, perhaps a lcal fracture and/r cnduit zne at the bttm f the Galena aquifer. As previusly mentined, the Glenwd Shale, the Platteville and Decrah frmatins cmprise an aquitard that separates the St. Peter Sandstne frm the Galena aquifer. T date, BS-1C cntains nly a few feet f water, suggesting that at this site, the aquitard has extremely lw cnductivity, at least when fractures and/r slutinal cnduits are nt encuntered. This als suggests that where unfractyred, the aquitard is very effective, affrding little leakage thrugh the rck matrix upward frm the St. Peter Sandstne t the Galena aquifer. Mnitring well BS-1D is pen t the St. Peter Sandstne. At this site, the St. Peter Sandstne acts as a cnfined aquifer and exists under flwing artesian cnditins; the head in this well is 6 t 7 feet (1.8 t 2.1 m) abve land surface, and it flws at abut 2 t 3 gallns per minute (7.6 t 11.4 liters per minute). This prtin f the basin is tpgraphically lwer than sites BS-2 thrugh BS-5. The bedrck als dips tward this area and grundwater is discharged thrugh Big Spring and assciated 17

21 Depth In meters On 20 Depth In feet ro Fm. 1 --Q. Mbr. SITE BS-3 z^^sz Z z / y / / r / Z /. / I If elev ft. (329 m) BS-3C H (a BS-3B Piezmeters -100 'lll'^ll1! / / / v / 7, 40 '-/W^/' ///ViV/',!,",','{','., I, I BS-3A PsaiiWsvsSL HH [^ N Sin-pit /III S.F. & 100 G-S. /_/ ; /_/ /_/ j /_ / / / / T~7 1 BS-3 Z ~7 7 III! Figure 12. Stratigraphic sequence and mnitring-well installatins at site BS-3 (cre lgged by M. J. Bunk).

22 Depth In Depth In meters feet 0-r Fm. Mbr. Oust SITE BS-4 Piezmeters elev ft. (355 m.) 20- d) C 0 O,» «<J «g H -P p : B= BS-4C "(1) Q- 40- uier 1 I 1 I III 7 / v/ / /./ BS-4BE O 3 (0 2 C ' LU * i \ i i > i i ' i ' i ' i ' i! ' i ' i-. ' ; ' ' ' ' Dub. 0 3 CO ' 1 ' 1^ 1 / / / III / 1 BS-4A 100- x: IM *1-1 1 M M C Q 6 a a 1 1 " A 1 y y y A _ _ ft _ 140- x: 2 8 D c 2 Gut. S.F Ptc> , 1 1, [ 1, 1 1, si BS-4 Figure 13. Stratigraphic sequence and mnitring-well installatins at site BS-4 (cre lgged by M. J. Bunk). 19

23 Table 1. Ptentimetric elevatins frm mnitring wells at sites BS-1 thrugh BS-4. Als shwn are fr matins screened and depth f screened interval belw land surface. MEAN WATER LEVEL DEPTH BELOW LAND SURFACE, IN FEET (METERS) WATER YEAR ( (+) feel abve land surface, ( ) n bservatins ) SCREENED PIEZOMETER FORMATION INTERVAL JAN FEB MAR APR MAY JUN JUL AUG SEP BS-1 A BS-1B BS-1C BS-1D QUATERNARY DUNLEITH PLATTEVILLE ST. PETER ( ) (4.29) (4.19) (3-94) (4.29) (4.31) (4.39) (4.51) (4.49) ( ) (2.73) (2.74) (2.41) (2.67) (2.78) (2.94) (3.00) ( ) ( ) (1.61) (1.39) (1.23) (4.38) BS-2A BS-2B BS-2C BS-2D BS-2E BS-2F BS-2G WISE LAKE DUNLEITH DUNLEITH DUNLEITH DUNLEITH PLATTEVILLE ST. PETER ( ) ( ) ( ) ( ) ( ) ( ) ( ) (46.92) (47.06) (56.07) (56.16) (47.03) (47.06) (56.18) (56.18) (56.25) (47.09) (47.18) (56.31) (56.46) (47.09) (47.07) (56.45) (56.48) BS-3C BS-3B BS-3A BS-3 QUATERNARY WISE LAKE DUNLEITH ST. PETER ( ) (4.66) (4.87) ( ) ( ) (51.16) (51.15) ( ) (91.43) (91.43) (5.00) (4.50) (4.67) (4.77) (4.81) (4.54) (51.17) (51.16) (51.15) (51.17) (91.46) (91.47) (91.46) (91.49) (91.68) BS-4C QUATERNARY ( ) (17.14) BS-4B QUATERNARY/ CLERMONT ( ) (22.23) BS-4A DUNLEITH/ WISE LAKE ( ) (65.51) (65.69) (65.79) (66.05) (66.32) (66.71) (66.90) (66.97) BS-4 ST. PETER ( ) (111.85) (112.04) (111.94) (112.05) (112.22) (112.46) (112.58) (112.75) 20

24 smaller springs t the Turkey River. Hydraulic head increases with depth, and the ptential fr grundwater flw is upward. Site BS-2 Figures 10 and 11 shw the stratigraphic sequence and mnitring-well installatins fr site BS-2. This site is lcated abut 100 feet (31 m) suth f Rbert's Creek, abve an inferred majr cnduit zne. In this prtin f the basin, Rbert's Creek and Silver Creek and their assciated alluvial valleys are 100 feet (31 m) r mre abve the Galena ptentimetric surface, which lies rughly in the middle f the aquifer. This site cntains seven mnitring wells (BS-2A thrugh BS-2G) (Table 1). BS-2A is screened int the middle f the Wise Lake Frmatin. BS-2B is screened int a 1 ft (0.31 m) thick vid within the Dunleith Frmatin. BS-2C is screened at apprximately the same depth as BS-2B, but did nt encunter the vid fund in BS-2B. BS-2D is als screened int the Dunleith Frmatin abut 9 feet (2.7 m) lwer than BS-2C. Mnitring wells BS-2A thrugh BS-2D have all remained essentially dry since cmpletin. BS-2E penetrates the same vid that was encuntered in BS-2B, and is screened int a larger vid apprximately 30 feet (9.1 m) lwer in the Dunleith Frmatin. This vid is apprximately 5 feet (1.5 m) thick and verlies a rubble zne that is als abut 5 feet (1.5 m) thick. The water level in BS-2E is abut 154 feet (47 m) belw land surface. BS-2F is pen t the Pecatnica Member f the Platteville Frmatin, within the aquitard which separates the Galena aquifer frm the St. Peter Sandstne aquifer. BS-2F has remained essentially dry since installatin, again suggesting extremely lw cnductivity within the matrix f the aquitard. Mnitring well BS-2G is pen t the St. Peter Sandstne. BS-2G penetrated the vid encuntered in wells BS-2B and BS-2E, but did nt penetrate the lwer vid which BS-2E is screened int. The water level in BS-2G rises frm the St. Peter t apprximately 185 feet (56 m) belw land surface, exhibiting cnfined aquifer cnditins. Since the head f the St. Peter aquifer is lwer than the head f the Galena aquifer, the ptential fr grundwater flw is dwnward frm the Galena t the St. Peter aquifer in this prtin f the basin. After BS-2E was partially cmpleted, water culd be heard discharging int the hle. T investigate this, a dwnhle camera was utilized, and water was seen discharging int the hle frm the upper vid and running dwn the side f the hle befre cmbining with water in the lwer vid. The upper vid appears t be a slutinally enlarged bedding plane. The mrphlgy f the lwer vid culd nt be determined, as its dimensins were greater than the camera lights culd penetrate. A small vlume f water was als nted seeping int the hle frm small fractures, vids and/r enlarged bedding planes at apprximately the same level as the bttm f the channel f Rbert's Creek. The dcumentatin f water seeping int BS-2E, cupled with bservatins that shw wells BS-2A thrugh BS-2D have remained essentially dry, suggest that Rbert's Creek is actively lsing water at this site. The distributin f vids in the mnitring wells demnstrates the cmplex distributin f fractures and/r vids within the Big Spring grundwater flw system. At this lcatin, sme water is cnducted alng minr unsaturated hrizntal passages frm the stream bed and shallw alluvial aquifer system befre reaching the saturated zne thrugh vertical fractures and vids. Water is als being channeled thrugh the upper slutinally enlarged bedding plane, abve the water table/ptentimetric surface. Passages such as these demnstrate the cmplexity f recharge-water flw paths assciated with the Galena aquifer. Site BS-3 Site BS-3 is lcated in the nrth-central prtin f the Big Spring basin near an area f sinkhle develpment (Figure 1). In this part f the basin the Galena ptentimetric surface is near, r abve the tp f the Galena carbnates (Figure 6). Figure 12 shws the stratigraphic sequence and mnitring-well installatins at site BS-3. At this lcatin, the Elgin Member f the Maquketa Frmatin, verlies a cmplete sequence f the Galena Grup. Mnitring well BS-3C is screened int the tp f the Elgin Member, and the verlying Quaternary depsits. Water levels 21

25 in BS-3C have been abut 15 feet (4.6 m) belw land surface, suggesting that at this site the tp f the Elgin Member and/r the verlying Quaternary depsits may be acting as an aquitard, allwing a shallw grundwater system t develp. Mnitring well BS-3B is pen int the tp f the Wise Lake Frmatin, and has remained essentially dry since installatin. This indicates that the Galena ptentimetric surface is belw the tp f the Galena aquifer at this lcatin. When BS-3A was drilled, the driller lst fluid in a weathered zne near the base f the Elgin Member at abut 42 feet (13 m). This suggests that there may be a zne f very high transmissivity lcated near the bttm f the Elgin Member and/r near the tp f the Galena aquifer at this lcatin. As drilling f BS-3A cntinued, the driller encuntered a cavity r rubble zne between 176 and 180 feet (54 and 55 m), within the Dunleith Frmatin. This cavity r rubble zne may be part f a cllapsed cnduit develped at this level. Mnitring well BS-3A is screened int the Dunleith Frmatin frm 165 t 185 feet (50 t 56 m), with the water level at abut 168 feet (51 m) belw land surface, exhibiting uncnfined aquifer cnditins. At this lcatin, the Galena ptentimetric surface was expected t be near, r abve the tp f the Galena aquifer, at an elevatin similar t the water level in mnitring well BS-3C (Figure 6). This site appears t be ver a cnduit-zne where the Galena ptentimetric surface is lcally depressed. This feature may nt be large enugh t have been reflected in previus assessments f water-level data. Mnitring well BS-3 is pen t the St. Peter Sandstne. The water level in BS-3 has been abut 300 feet (91 m) belw land surface, exhibiting cnfined aquifer cnditins. Since heads shw a decrease with depth, in this prtin f the basin, ptential fr grundwater mvement is dwnward frm the Galena t the St. Peter aquifer. Site BS-4 Site BS-4 is lcated in the nrthwest prtin f the basin, near the western edge f the basin (Figure 1). In this area, Galena carbnates are verlain by the Maquketa Frmatin, and the Galena ntentimetric surface is lcated abve the tp f the Galena aquifer (Figure 6). Figure 13 shws the stratigraphic sequence and mnitring-well installatins fr site BS-4. At this site, Pleistcene sediments verlie the Clermnt and Elgin members f the Maquketa Frmatin, which in turn, verlie a cmplete sequence f the Galena Grup. Mnitring well BS-4C is screened int uncnslidated, stratified Pleistcene depsits and has a water level apprximately 56 feet (17 m) belw land surface. Mnitring well BS-4B is pen t the Clermnt Member, with a water level at abut 76 feet (23 m) belw land surface. When BS-4B was drilled, the driller nted the lss f fluid near the bttm f the Clermnt Member, at arund 139 feet (42 m). Well BS-4A is pen t abut the middle f the Galena aquifer, with a water level apprximately 217 feet (66 m) belw land surface, exhibiting cnfined aquifer cnditins. During the drilling f BS-4A, the driller nted the lss f fluid at abut 100 feet (31 m), and again at abut 134 feet (41 m), within the Pleistcene depsits verlying the Clermnt Member. Mnitring well BS-4 is pen t the St. Peter Sandstne, with a water level abut 367 feet (112 m) belw land surface, exhibiting cnfined aquifer cnditins. During the drilling f BS-4, the driller lst fluid just abve the Clermnt Member at 132 t 134 feet (40 t 41 m), and after casing t 149 feet (45 m), again lst fluid at 151 feet (46 m). As drilling f BS-4 cntinued, n vids were encuntered within r belw the Galena aquifer. The lss f fluid, bth abve and immediately belw the Clermnt Member, in cnjunctin with bservatins f differing water levels in mnitring wells BS-4B and BS-4C, suggest that there may be shallw flw systems develped within the Maquketa Frmatin and the verlying Pleistcene depsits at this lcatin. Cnversely, the apparent lack f vids within the Galena aquifer at this lcatin, suggest that the verlying Maquketa Frmatin limits the develpment f slutin cnduits in this prtin f the Big Spring basin. The ptential fr grundwater mvement is dwnward in this prtin f the basin, since heads decrease with depth at this site. 22

26 Site BS-5 Site BS-5 was drilled t btain a mre cmplete sequence f the Maquketa Frmatin than was present at site BS-4. N mnitring wells were installed at this lcatin. WELL MONITORING INSTALLATIONS T allw cntinuus mnitring f water levels within the wells, digital stage recrders, driven by flat and tape assemblies and pwered by 12 vlt gell cell batteries were installed. The recrders are cntrlled by slid-state timers, set t register water levels at 1 hur intervals. The recrders are hused in wden bxes, munted n the well casings. The digital tapes are remved n a weekly basis by USGS persnnel and the data is prcessed and stred in the USGS, Autmatic Data Prcessing System (ADAPS) data base. The mnitring wells that have remained essentially dry since installatin were nt instrumented. Mnitring wells were cnstructed as cnsistently as pssible using similar installatin techniques, and materials. All wells were gruted abve the screens with bentnite and cement. At site BS-1, mnitring wells BS-1 A and BS-1B are cnstructed with 4 inch (10.2 cm) steel casing, and are instrumented with digital recrders. BS-1C is cnstructed with 6 inch (15.2 cm) steel casing t 61 feet (19 m), then 4 inch (10.2 cm) steel casing t 150 feet (46 m). BS-1D, which is a flwing artesian well, is cnstructed with 6 inch (15.2 cm) steel casing t 61 feet (19 m), and 4 inch (10.2 cm) steel casing frm 47 feet t 173 feet (14 m t 53 m). It is fitted with a pressure gage and dial. Dial readings are mnitred by GSB and USGS persnnel, and are cnverted t feet f head by a cnversin factr. At site BS-2, mnitring wells BS-2A/BS-2B and BS-2C/BS-2D are cnstructed with 2 inch (5.1 cm) PVC casing, nested inside f 6 inch (15.2 cm) steel casing. BS-2F is cnstructed with 4 inch (10.2 cm) steel casing. Mnitring wells BS-2E and BS-2G are cnstructed with 4 inch (10.2 cm) steel casing, and are instrumented. At site BS-3, mnitring wells BS-3C and BS-3B are cnstructed with 4 inch (10.2 cm) steel casing. BS-3A is cased t 42 feet (13 m) with 6 inch (15.2 cm) steel casing, then with 4 inch (10.2 cm) steel casing t 198 feet (60 m). BS-3A and BS-3C are instrumented fr cntinuus mnitring. Mnitring well BS-3 is cased t 41 feet (13 m) with 6 inch (15.2 cm) steel casing, then with 4 inch (10.2 cm) steel casing t 351 feet (107 m). Instrumentatin f well BS-3 has been replaced by mnthly measurement by USGS persnnel. At site BS-4, all wells are instrumented fr cntinuus mnitring. BS-4C is cnstructed with 4 inch (10.2 cm) steel casing. BS-4B is cased t 76 feet (23 m) with 5 inch (12.7 cm) steel casing, then t 130 feet (40 m) with 4 inch (10.2 cm) steel casing. BS-4A is cased t 134 feet (41 m) with 5 inch (12.7 cm) steel casing, then 4 inch (10.2 cm) steel casing t 261 feet (80 m). Mnitring well BS-4 is cased t 133 feet (41 m) with 6 inch (15.2 cm) steel casing, then 4 inch (10.2 cm) steel casing t 550 feet (168 m). PRELIMINARY WATER-LEVEL DATA (January 1989 t September 1989) As previusly mentined, 1988 and 1989 were tw f the driest cnsecutive years in Iwa's recrded histry; state-wide, average precipitatin was mre than 18 inches (45.7 cm) belw nrmal. Precipitatin in the Big Spring area was abut 10 inches (25.4 cm) belw nrmal during the 1988 Water Year and abut 8.7 inches (22.1 cm) belw nrmal during the 1989 Water Year. Frm January 1989 t September 1989, all mnitring wells exhibited an verall decline f mean mnthly water levels, reflecting the cntinued absence f recharge during this drught perid. The shallwer wells exhibited mre immediate respnses t snw melt and rain fall events than the deeper mnitring wells. At site BS-1 all wells exhibited verall declining mean mnthly water levels (Table 1). Mnitring wells BS-1 A and BS-1B rse in respnse t snw melt in February, and BS-1A als shwed an increase in August in respnse t rain fall. Mean mnthly water levels in BS-1 A declined frm feet (4.29 m) in January t feet (4.34 m) in September. The largest change in mean daily water levels in BS-1A was a 23