Effect of nitrate, organic carbon, and temperature on potential denitrification rates in nitrate-rich riverbed sediments

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1 University f Nebraska - Lincln DigitalCmmns@University f Nebraska - Lincln USGS Staff -- Published Research US Gelgical Survey 1996 Effect f nitrate, rganic carbn, and temperature n ptential denitrificatin rates in nitrate-rich riverbed sediments K. S. Pfenning U.S. Gelgical Survey P. B. McMahn U.S. Gelgical Survey Fllw this and additinal wrks at: Part f the Earth Sciences Cmmns Pfenning, K. S. and McMahn, P. B., "Effect f nitrate, rganic carbn, and temperature n ptential denitrificatin rates in nitrate-rich riverbed sediments" (1996). USGS Staff -- Published Research This Article is brught t yu fr free and pen access by the US Gelgical Survey at DigitalCmmns@University f Nebraska - Lincln. t has been accepted fr inclusin in USGS Staff -- Published Research by an authrized administratr f DigitalCmmns@University f Nebraska - Lincln.

2 ELSEVER Jurnal f Hydrlgy 187 (1996) Jurnal f Hydrlgy Effect f nitrate, rganic carbn, and temperature n ptential denitrificatin rates in nitrate-rich riverbed sediments K.S. Pfenning*, P.B. McMahn us Gelgical Survey, Denver Federal Center, Mail Stp 415, Denver, CO 80255, USA Received 7 Nvember 1995; revised 13 February 1996; accepted 15 February 1996 Abstract A study cnducted in 1994 as part f the US Gelgical Survey's Natinal Water-Quality Assessment Prgram, Suth Platte River Basin investigatin, examined the effect f certain envirnmental factrs n ptential denitrificatin rates in nitrate-rich riverbed sediments. The acetylene blck technique was used t measure nitrus xide (N20) prductin rates in labratry incubatins f riverbed sediments t evaluate the effect f varying nitrate cncentratins, rganic carbn cncentratins and type, and water temperature n ptential denitrificatin rates. Sediment incubatins amended with nitrate, at cncentratins ranging frm 357 t 2142 ~tml 1 - (as measured in the field), prduced n significant increase (P > 0.05) in N20 prductin rates, indicating that the denitrificatin ptential in these sediments was nt nitrate limited. n cntrast, incubatins amended with acetate as a surce f rganic carbn, at cncentratins ranging frm 0 t 624 ~ml 1-1, prduced significant increases (P < 0.05) in N20 prductin rates with increased rganic carbn cncentratin, indicating that the denitrifieatin ptential in these sediments was rganic carbn limited. Furthermre, N20 prductin rates als were affected by the type f rganic carbn available as an electrn dnr. Acetate and surface-water-derived fulvic acid supprted higher N20 prductin rates than grundwater-derived fulvic acid r sedimentary rganic carbn. Lwering incubatin temperatures frm 22 t 4 C resulted in abut a 77% decrease in the N20 prductin rates. These results help t explain findings frm previus studies indicating that nly 15-30% f nitrate in grundwater was denitrified befre discharging t the Suth Platte River and that nitrate cncentratins in the river generally were higher in winter than in summer. 1. ntrductin The US Envirnmental Prtectin Agency has set the maximum cntaminant level (MCL) fr nitrate in drinking water at 10 mg l -t as nitrgen (714 ~tml l-t). Grundwater * Crrespnding authr $ Elsevier Science B.V. All rights reserved P S (96)

3 284 K.S. Pfenning, P.B. McMahnJurnal f Hydrlgy 187 (1996) mgmtd had Alluvium buada~! 0M 2M ', _ Clrad! N - 2~ Stu a? L\ ~ f " J At ~ Gilcrest e" J Fig. 1. Lcatin f study area. in agricultural areas ften cntains nitrate in excess f the MCL wing t anthrpgenic surces such as fertilizers, septic tank drainage, feedlts, and dairy and pultry farming (Madisn and Brunett, 1984; Gld et al., 1990; Buchard et al., 1992). At cncentratins abve the MCL, nitrate can be txic t infants and pse ther human health prblems (Buchard et al., 1992). n additin, nitrate-cntaminated grundwater discharging t surface water can degrade water quality by prmting eutrphicatin f surface water. A buildup f nitrate in the grundwater f the Suth Platte alluvial aquifer in Clrad has ccurred because f applicatin f nitrgen t fields and leaching f nitrgen by irrigatin water (Austin, 1993; Wylie et al., 1993; McMahn and B6hlke, 1996) (Fig. 1). n these areas f irrigated agriculture, discharge frm the aquifer is a majr cntributr t flw in the Suth Platte River (Rbsn, 1989). Measured rates f grundwater discharge ranged frm t as high as m 3 s -1 km -l during the irrigatin seasn (Ruddy, 1984). When substantial quantities f grundwater having elevated nitrate cncentratins discharged t the river, nitrate cncentratins in the river increased (McMahn and B6hlke, 1996). Earlier studies indicated that denitrificatin in sediments underlying and adjacent t the river remved a part f the nitrate frm grundwater befre its discharge t the river (McMahn and B6hlke, 1996). Hwever, thse studies did nt identify the factrs affecting denitrificatin, such as nitrate cncentratin, rganic carbn availability, and temperature (Smith and Duff, 1988; Mrris et al., 1988; Fujikawa and Hendry, 1991; Smith et al., 1991; Krm, 1992; Starr and Gillham, 1992; Bradley et al., 1992, 1995; Hansn et al., 1994).

4 K.S. Pfenning, P.B. McMahrdJurnal f Hydrlgy 187 (1996) n 1994, as part f the US Gelgical Survey Natinal Water-Quality Assessment (NAWQA) prgram, Suth Platte River Basin investigatin, a study was cnducted t examine the effect f several envirnmental factrs n denitrificatin rates. This paper presents the results f that investigatin, in which the effects f varying nitrate cncentratins, rganic carbn cncentratins and type, and water temperature n ptential rates f denitrificatin in riverbed sediments were evaluated. 2. Study area The study area is lcated in a part f the Suth Platte River Basin between Gilcrest and Greeley, Clrad (Fig. 1). The alluvial aquifer in this area cnsists f uncnslidated sand and gravel f Quaternary age with minr amunts f interbedded clay and silt. Relatively impermeable fine-grained sediments f Cretaceus age underlie the aquifer. These Cretaceus sediments yield little water t the aquifer. rrigatin water taken frm the Suth Platte River and frm the alluvial aquifer is applied t fields verlying the alluvium and prvides the main surce f recharge fr the aquifer. The aquifer in the study area has a saturated thickness f abut m. Grundwater in the study area generally flws t the west-nrthwest tward the fiver. The Suth Platte River at the study area is mderately braided, apprximately 50 m wide, and has an average depth f 0.4 m (McMahn and 136hlke, 1996). Grundwater cllected frm multi-depth piezmeters and wells screened belw and adjacent t the fiver in the study area had nitrate cncentratins ranging frm abut 390 t 2000 ~ml 1-1, whereas the nitrate cncentratins in the fiver ranged frm 250 t 400 t~ml 1-1 (McMahn and Bfhlke, 1996). Cncentratins f disslved rganic carbn (DOC) in grundwater ranged frm 230 t 310 tlml 1-1 and ne fiver-water sample cntained 433~ml 1-1. Measured grund-water temperatures ranged frm 2 t 18.1 C, and fiver-water temperatures ranged frm 4.5 t 21.5 C. Disslved xygen (DO) cncentratins in the grundwater frm the study area were less than 20 t~ml! -1. The nitrate lad, based n mass balance, added t the fiver frm grundwater discharge in Nvember 1993 was 230 kg N day -1 km -] (McMahn and B6hlke, 1996). 3. Methds 3.1. Sediment sampling Samples f riverbed sediment were cllected frm the sediment-water interface (surficial sediments) and frm three depths belw the interface (buried sediments) t examine the effect f nitrate and rganic carbn cncentratins n ptential denitrificatin rates in varius sediments underlying the fiver. Riverbed sediment was scped frm the sediment-water interface directly int jars and then refrigerated fr as much as 2 weeks until experiment began in the labratry. Three different surficial sediments were cllected based n grain size: fine-, medium-, and carse-grained sediments (Table 1). Riverbed sediments belw the sediment-water interface were extracted, using a peristaltic pump,

5 286 KS. Pfenning, P.B. McMahnJ urnal f Hydrlgy 187 (1996) Table 1 Organic carbn cntent and grain size f sediment sampled Sediment sample Sedimentary rganic Median grain carbn cntent (wt %) size (ram) Fine-grained surficial Medium-grained surficial Carse-grained surficial 0.61 m belw sediment-water interface 1.22 m belw sediment-water interface 2.44 m belw sediment-water interface frm a drive pint with a screen f cm length with penings f 3 nun diameter. The drive pints were hammered int the sediments underlying the river at depths f 0.61, 1.22, and 2.44 m belw the sediment-water interface; the drive pints were remved after sediment cllectin. This cllectin technique excluded any sediment with grain size greater than 3 ram Slutin preparatin The riverbed sediments were drained t decrease disslved cncentratins f nitrate and rganic carbn initially present in the sediment. Once the sediments were drained, varying amunts f nitrate were added t them t analyze the effect f nitrate cncentratin n ptential denitdficatin rates. Nitrate slutins were made using sdium nitrate and deinized water. The surficial sediments were nly incubated in slutins cntaining 357~ml 1-1 nitrate because f the narrw range f nitrate cncentratins measured in the river (McMahn and B6hlke, 1996). The buried sediments were incubated in slutins cntaining 357, 1071 r 2142~ml 1-1 nitrate. These cncentratins cver the range f nitrate cncentratins measured in grundwater at the site. Slutins cntaining varying amunts f acetate were added t the sediments in separate experiments t evaluate the extent t which ptential denitrificatin rates were limited by rganic carbn availability. All sediments, except fr the fine-grained surficial sediments, were incubated with slutins cntaining 0, 208 r 624zml 1-1 rganic carbn as acetate. The fine-grained surficial sediment was incubated with a slutin cntaining either 0 r 208~ml C 1-1 as acetate because f its relatively high sedimentary rganic carbn cntent. The initial nitrate cncentratin was 357 ~tml 1-1 fr all incubatins used t analyze the effect f rganic carbn availability n denitrificatin rates. Separate experiments were perfrmed t examine the effect f rganic carbn type n ptential rates f denitrificatin by adding 208zml 1-1 rganic carbn as fulvic acid derived f~m surface water r grundwater (Aiken and Malclm, 1987) r 208 ~,ml l- rganic carbn as acetate t the medium-grained surficial sediment. The medium-grained surficial sediment was chsen because f its relatively lw sedimentary rganic carbn cntent (Table 1). All sediments initially were incubated at 22 C. Additinal experiments were run at 4 C, using the medium-grained surficial sediment t analyze the effect f temperature n

6 K.S. Pfenning, P.B. McMahnJurnal f Hydrlgy 187 (1996) ptential denitrificatin rates. Fr these experiments, a slutin cntaining 357~ml 1-1 nitrate and either 0 r 208 ~tml 1-1 rganic carbn, as acetate, was used Measuring ptential denitrificatin rates Sediments were incubated in 120 ml serum bttles. Each bttle cntained 40 g f mist sediment amended with 50 ml f slutin, leaving 55 ml f headspace. T decrease xygen levels, the sediment and slutin then were sparged with xygen-free nitrgen gas fr 5 rain and immediately capped with thick butyl stppers. T begin incubatin, 2.5 ml (5% f the headspace) f acetylene was injected. The acetylene was generated by adding deinized water t calcium carbide. Acetylene blcks the cnversin f N20 t N2; studies have indicated that N20 prductin can be used t measure ptential denitrificatin rates (Klemedtssn et al., 1988; Mrris et al., 1988; Fujikawa and Hendry, 1991; Bradley et al., 1992). All f the sediment-slutin cmbinatins were run in triplicate with a furth bttle as a cntrl. The cntrl bttles were prepared as the thers but were autclaved fr 20 rain at 120 C t render denitrifying bacteria inactive befre acetylene additin, and thus acted as a baseline fr abitic N20 prductin. The cntrls did nt prduce N20 during the experiments. The headspace in the vials was sampled at fur r five time steps beginning at time zer. Samples were taken by inserting a 1 rni pressure-lck glass syringe int the butyl stpper and extracting gas frm the headspace. Each sample was analyzed fr N20 and CO2, using a gas chrmatgraph with a thermal cnductivity detectr. N20 and CO2 cncentratins were adjusted fr the gas in slutin using Bunsen slubility cefficients (Weiss and Priee, 1980) and Henry's law cnstants. Frm these cncentratins, the prductin f NzO and CO2 was quantified ver time by perfrming a linear regressin n the time-series data. The relatin between rates f denitrificatin and nitrate r rganic carbn cncentratins was analyzed by linear-regressin analysis. Levels f significance were set at ~ fr all statistical analyses. Once the experiments were cmpleted, sediments were ven-dried befre weighing, carbn analysis, and grain-size analysis. Organic carbn was calculated by subtracting inrganic carbn frm ttal carbn, bth f which were analyzed using a culmeter. 4. Results and discussin 4.1. Effect f nitrate and rganic carbn cncentratins n N~ prductin rates Rates f N20 prductin did nt increase significantly (P > 0.05) with increasing nitrate cncentratin in any buried sediments (Fig. 2). These results indicated that the denitrificatin ptential was nt limited by the availability f nitrate, which is cnsistent with the relatively high cncentratins f nitrate measured in grundwater beneath the river at this site ( ~ml 1-~; McMahn and B6hlke, 1996)). n cntrast, there were significant increases (P < 0.05) in N20 prductin rates with increasing acetate cncentratins in fur f six incubatins (Fig. 3). The exceptins--the fine-grained surficial sediment and the

7 288 KS. Pfenning, P.B. McMahnJurnal f Hydrlgy 187 (1996) O O E t =.9 -t "U = Q z T m~ 4 "~. ~. -. ~- Sample depths --= m m 2.44 m... i 1. _j_ 0.004,,, 1 t, Nitrate (l~mvl) Fig. 2. Relatin f N20 prductin rate t nitrate cncentratin in incubatins f buried riverbed sediments. All sediments were incubated withut additin f rganic carbn. Pints represent average f triplicate measurements ±SD. buried sediment frm the 0.61-m depth did nt indicate a significant increase (P > 0.05) in N20 prductin rates with increasing acetate cncentratin. Fr the fine-grained surficial sediment, the lack f increase was prbably due t the relatively high rganic carbn cntent f the sediment (Table 1). Overall, these results indicated that the denitrificatin ptential in mst sediments was limited by the availability f rganic carbn. Other studies als have shwn that lw sedimentary rganic carbn cntents may limit ptential denitrificatin rates (Smith and Duff, 1988; Mrris et al., 1988; Hammn and Fustec, 1991; Start and Gillham, 1992; Bradley et al., 1992, 1995). A cnsequence f the rganic carbn limitatin in the Suth Platte alluvial aquifer system may be that nitrate in discharging grundwater was nly partly denitrified befre entering the river. McMahn and B6hlke (1996) estimated that 15-30% f the nitrate in grundwater at this site was denitrified befre discharging t the river. Cmparisn f ptential N20 prductin rates measured under in situ nitrate and rganic carbn cncentratins with cncentratins f nitrate and rganic carbn measured in the field (Fig. 4) further supprts the cnclusin that denitrificatin ptential in the riverbed sediments is generally limited by the availability f rganic carbn. The highest ptential rates f N20 prductin ccurred in surficial sediments cntaining relatively high rganic carbn cntents even thugh stream-water nitrate cncentratins were lw (Fig. 4). n cntrast, lwer rates f N20 prductin ccurred in buried sediments having less rganic carbn, but higher nitrate cncentratins.

8 K.S. Pfenning, P.B. McMahnlJurnal f Hydrlgy 187 (1996) , O J t ~ T- E -1 "" 0.08 U u 0.06 Z 0.04 (a) ± - Surficial sediments Grain size. --'-- Free Mcdiun,-*, Carse E = 0.08.~, O "S 0.06 t2. % 0.04 Z 0.02 iii (b).,, ' ' ' ' Buried sediments Acetate (ttml1) ± Depth r~ ---v r~ - - 2:44m Fig. 3. Relatin f N20 prductin rate t acetate cncentratin (a) fr surficial riverbed sediments and (b) fr buried riverbed sediments. All sediments were incubated with 357 ~mi 1 -l nitrate. Pints represent average f triplicate measurements ± 1 SD.

9 i 290 K.S. Pfenning, P.B. McMahnlJurnal f Hydrlgy 187 (1996) O.& 1~ _.-..~ Hi. Ca) O0 "Hi i t ~, i, i 2 i t i 3 D.N =,~ 2OOO a i. j O in (b) p,\ i i i i i, 1 2! 400 t \\\" \ \ \ 25O Cc) 2OO ,!, 2 (d) " t 0, i! 1 2 Depth be~wsedunentw~er interface (m)

10 KS. lffenning, P.B. McMahnJurnal f Hydrlgy 187 (1996) i Acetate r 2 = 0.99 t E.16 ::::L Q) L- e e-~ 0.12 Z Surface-w fulvic acid G~undwater-defulvic acid N rganic carbn Ttal CO 2 prductin (gml d" l g- l ) Fig. 5. Relatin f N20 and ttal CO 2 prductin rates t types f rganic carbn. Medium-grained riverbed sediment was incubated using 208 #ml 1-1 rganic carbn fr all incubatins, except the incubatin having n added rganic carbn. Pints represent average f triplicate measurements ± 1 SD Effect f rganic carbn type n N20 prductin rates Rates f N20 prductin were related t the type f rganic carbn present in the sediments (Fig. 5). The highest N20 prductin rates were in incubatins amended with acetate, and the lwest rates were in incubatins with n added rganic carbn. Rates prduced in incubatins amended with fulvic acids were intermediate. Acetate is a relatively labile frm f rganic carbn s it is nt surprising that the highest N20 prductin rates were in the acetate-amended incubatins. Hwever, high cncentratins f acetate Fig. 4. Relatin f depth t (a) N20 prductin rate, (b) disslved nitrate cncentratin, (c) disslved rganic carbn cncentratin, and (d) sedimentary rganic carbn cntent. Nitrus xide prductin rates are prducts f incubatin with nitrate cncentratin which crrespnd t- cncentratins measured in the field and with n additin f rganic carbn.

11 292 K.S. Pfenning, P.B. McMahnJurnal f Hydrlgy 187 (1996) '~u 0.18 "7 " E -..1 "g 0.14 = 0.12 O r,~ = 0.10 O ~ % Z 0.06 J i F-- ~,~,~e~ ft_ (a) E 250 r 2 = r., u 50 Z! 1, (b) 5 i 1 t Temperature (*C) 30 Fig. 6. Relatin f water temperature t (a) NzO prductin rate in incubatins f medium-grained riverl~ sediments using 357 tlml 1-1 nitrate, and (b) nitrate cncentratin in the Suth Platte River at Julesburg, Clrad. Water-quality data frm Ugland et al. (1994). generally were nt present in shallw grundwater. The surface-water-derived fulvic acid als supprted relatively high N20 prductin cmpared with the grundwater-derived fulvic acid (Fig. 5). These data indicated that shallw riverbed sediments subjected t mixing with surface water may supprt higher denitrificatin rates than the deeper

12 K.S. Pfenning, P.B. McMahnJurnal f Hydrlgy 187 (1996) riverbed sediments because f the type f rganic carbn available as an electrn dnr. The CO2 prductin rates in the incubatins als were related t the type f rganic carbn in the sediment, but were inversely related t the N20 prductin rates (Fig. 5). The inverse relatin between N20 and CO2 prductin rates may indicate that the different rganic substrates used in the incubatins vary in xidatin state. Fr example, if rganic carbn in the fulvic acids had a higher xidatin state than carbn in acetate, the fulvic acid wuld require greater carbn turnver per mle f nitrate reduced. Assuming that the CO~q'q20 prductin rati is a measure f the efficiency by which the different rganic substrates supprted denitrificatin, then prgressively less efficient denitrificatin reactins ccurred in incubatins amended with acetate, surface-water-derived fulvic acid, and grundwater-derived fulvic acid (Fig. 5). The relative efficiency f different types f rganic carbn in supprting denitrificatin may be imprtant in systems where denitrificatin is rganic carbn limited Effect f temperature n NzO prductin rates Rates f N20 prductin decreased by as much as 77% in sediments incubated at 4 C cmpared with the same sediments incubated at 22 C (Fig. 6(a)). These results are cnsistent with the findings frm previus studies indicating that denitrificatin rates were affected by seasnal temperature patterns (Hansn et al., 1994). An imprtant cnsequence f these findings is that nitrate cncentratins in reaches f the Suth Platte River dminated by grundwater discharge may increase during the winter when lwer temperatures suppress denitrificatin. Nitrate and temperature data were cllected ver a 22 mnth perid frm the Suth Platte River at Julesburg, Clrad, a site with sediment similar t the study area. Flw in the Suth Platte River in the Julesburg area is almst 100% grundwater return flw. An inverse relatin was bserved between nitrate and temperature (Fig. 6(b)), indicating that lwer temperatures may have suppressed denitrificatin in sediments alng that reach. These results suggest that the chemistry f the river water seems t be at least partly cntrlled by micrbial prcesses in the riverbed sediments that reduced nitrate cncentratins in grundwater befre it discharged t the river. 5. Cnclusin Denitrificatin in riverbed sediments is a sink fr nitrate in grundwater that discharges t the Suth Platte River. Labratry experiments indicated that ptential rates f denitrificatin in the riverbed sediments were limited by the availability f rganic carbn and by temperature. Ptential denitrificatin rates were nt nitrate limited. Acetate and surface-water-derived fulvic acid supprted higher N20 prductin rates than grundwater-derived fulvic acid r sedimentary rganic carbn. Ptential rates f denitrificatin measured at 4"C were abut 77% lwer than rates measured at 22 C in the same sediment. Results frm this study indicated that high nitrate cncentratins in segments f the Suth Platte River, which are cntrlled by grundwater discharge, may be due in part t

13 294 K.S. lffenning, P.B. McMahnJurnal f Hydrlgy 187 (1996) denitrificatin in riverbed sediments that is limited by rganic carbn availability. Relatively high nitrate cncentratins in the river during winter may indicate that lw temperatures seasnally suppress denitrifying activity in riverbed sediments. Acknwledgements This study was funded by the US Gelgical Survey Natinal Water-Quality Assessment (NAWQA) prgram, Suth Platte River Basin Study Unit. We thank Tim Cwdery, Larry PuckeR, and reviewers at the jurnal fr their helpful cmments n earlier versins f this manuscript. References Aiken, G.R. and Malclm, R L., Mlecular weight f aquatic fulvic acids by vapr pressure smmetry. Gechim. Csmchim. Acta, 51: Austin, B., Grundwater mnitring activities Suth Platte River alluvial aquifer Reprt t the Cmmissiner f Agriculture, Clrad Department f Agriculture. Buchard, D.C., Williams, M.K. and Surampalli, R.Y., Nitrate cntaminatin f grundwater: surces and ptential health effects. J. Am. Water Wrks Assc., September 1992: 84(9): Bradley, P.M., Aelin, C.M. and Vrblesky, D.A., nfluence f envirnmental factrs n denitrificatin in sediment cntaminated with JP-4 jet fuel. Grundwater, 30: Bradley, P.M., McMahn, P.B. and Chapelle, F.H., Effects f carbn and nitrate n denitrificatin in bttm sediments f an effluent-dminated river. Water Resur. Res., 31: Fujikawa, J.l. and Hen&y, M.J., Denitrificatin in clayey till. J. Hydrl., 127: Gld, A.J., DeRagn, W.R., Sullivan, W.M. and Lemunyn, J.L., Nitrate-nitrgen lsses t grundwater frm rural and suburban land uses. J. Sil Water Cnserv., 45: Hammn, M. and Fustec, E., Labratry and field study f an in situ grundwater denitrificatin reactr. Res. J. Water PHut. Cntrl Fed., 63: Hansn, G.C., Grffman, P.M. and Gld, A.J., Denitrificatin in riparian wetlands receiving high and lw grundwater nitrate inputs. J. Envirn. Qual., 25: Klemedtssn, L., Svenssn, B.H. and Rsswall, T., A methd f selective inhibitin t distinguish between nitrificatin and denitrificatin as surces f nitrus xide in sil. Bil. FertiL Sils, 6: Krm, S.F., 1992, Natural denitrificafin in the saturated zne: a review. Water Resur. Res., 28: Madisn, R.J. and Brunett, J.O., Overview f the ccurrences f nitrate in grundwater f the United States. US Gelgical Survey Natinal Water Summary US Gelgical Survey, Denver, CO, pp McMahn, P.B. and B6hlke, J.K., Denitrificatin and mixing in a streamaquifer system: effects n nitrate lading t surface water. J. Hydri., 186: Mrris, J.T., Whiting, G.J. and Chapelle, F.H., Ptential denitrificatin rates in deep sediments frm the sutheastern Castal Plain. Envirn. Sci. Technl., 22: Rbsn, S.G., Alluvial bedrck aquifers f the Denver Basin; eastern Clrad's dual grund-water resurce. US Gel. Surv. Water-Supply Pap., 2302, 40 pp. Ruddy, B.C., Streamflw gain-and-lss and suspended-sediment characteristics f the Suth Platte River and three canals near Frt Mrgan, Clrad. US Gel. Surv. Water-ResuL nvest. Rep., , 82 pp. Smith, R.L. and Duff, J.H., Denitrificatin in a sand and gravel aquifer. Appl. Envirn. Micrbil., 54: Smith, R.L., Hwes, B.L. and Duff, J.H., Denitrificatin in nitrate-cntaminated grundwater: ccurrence in steep vertical gechemical gradients. Gechim. Csmchim. Acta, 55: Starr, R.C. and Oiltham, R.W., Denitrificatin and rganic-carbn availability in tw aquifers. Grundwater, 31:

14 CS. Pfenning. P.B. McMahnJurnal f Hydrlgy 187 (1996) Ugland, R.C., Cchran, BJ., Hiner, M.M. and Steger, R.D., Water Resurces Data, Clrad, Water Year 1993, Vl.. US Gelgical Survey Water-Data Reprt N. CO US Gelgical Survey, Denver, CO. Weiss, R.F. and Price, B.A., Nitrus xide slubility in water and seawater. Mar. Chem., 8: Wylie, B.C, Wagner, D.G., Hffer, R.M., Maxwell, S. and Shaffer, MJ., Spatial distributin f nitrate leaching 'ht spts' and nitrate cntributins t the Suth Platte River Basin aquifers. Rep. 181, Clrad Water Resurces Research nstitute, Frt Cllins, CO, 12 pp.