In situ measurements and numerical simulation of nitrate leaching from farm soil

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1 Calibratin and Reliability in Grundwater Mdelling (Prceedings f the MdelCARE 99 Cnference held at Zurich, Switzerland, September 1999). IAHS Publ. n. 265, In situ measurements and numerical simulatin f nitrate leaching frm farm sil N. YAMASHITA & S. SUGIO Graduate Schl f System Engineering, Miyazaki University, 1-1 Miyazaki , japan stigi@civil.niiyazaki-u.ac.jp Gakuen-kibanadai-Nishi, Abstract The uncnfined aquifers in the suthern part f Kyushu Island in Japan are cntaminated by N0 3 -N. It is cnsidered that the cntaminatin is caused by seepage f rainwater with N0 3 -N disslved frm the barnyard manure n the farms. T investigate NO3-N leaching in the surface sil at an actual farm, in situ measurements f N0 3 -N cncentratins in sil misture within 3 m f the grund surface were carried ut. Mrever, ne-dimensinal NO3-N leaching frm sil was calculated numerically t simulate the bserved leaching. Frm these results, it was clarified that N0 3 -N disslved frm the barnyard manure and the fertilizer was leaching in the surface sil at high cncentratins. INTRODUCTION The suthern part f Kyushu Island in Japan has ver 2500 mm f annual precipitatin and als cnsists f thick aquifers with high permeability. All drinking water resurces in this part depend n the grundwater resurces. Hwever, grundwater cntaminatin by NO3-N has been detected. Accrding t previus investigatins, ne f the causes f this cntaminatin was ver-use f barnyard manure n the farms (Gillham & Webber, 1969; Yamashita & Sugi, 1999). The requirement fr grundwater cnservatin in this part f the island is t understand NO3-N perclatin frm faims. Studies using numerical mdels were intrduced in several fields but nly sme f them were validated against in situ measurement data. In this study, in situ measurements f NO3-N cncentratin were carried ut, and these results were simulated apprximately by means f numerical simulatin. NO3-N CONCENTRATIONS OF SOIL MOISTURE IN THE FARM NO3-N cncentratins f sil misture within 3 m f the grund surface were measured at a farm in the study area. Prus cups were used t cllect the sil misture. They were arranged in tw rws: a west rw and an east rw. In each rw, prus cups were buried at depths f 25, 50, 75, 100, 150, 200, 250 and 300 cm in the sil as shwn in Fig. 1. In situ measurements were started in April 1998 and frage crps were cultivated n the farm. Table 1 shws the applicatin dates and the quantities as nitrgen f bth the manure and the chemical fertilizer, and Fig. 2 shws daily rainfall. Frm the measurements, it was fund that high cncentratins f NO3-N were detected as shwn in Fig. 3 and the maximum NO3-N cncentratin value

496 N. Yamashita & S. Sugi reached 280.8 mg 1" at 50 cm depth. The vertical distributins f NO3-N cncentratin measured n 21 July and 11 September seem t have nt changed (Fig. 3).

2 496 N. Yamashita & S. Sugi reached mg 1" at 50 cm depth. The vertical distributins f NO3-N cncentratin measured n 21 July and 11 September seem t have nt changed (Fig. 3). This shws that NO3-N leaching depends strngly n rainfall cnditins. I g i Grund surface 25cm A n d sul I'rir- cup - > M r a ".;>,! 300c nr" Silt G r u n d w a t e r l e v e l Fig. 1 Schematic diagram f sil misture samplers. ± S Z Table 1 Applicatin dates and quantities f manure and fertilizer nitrgen applied t the farm. Date 1 March April April May August 1998 Activity in the farm Manuring Seeding Fertilizing Fertilizing Harvest Nitrgen applied (kg ha" 1 ) Manuring Seeding Fertilizing Fertilizing Harvest L 1 y j. \ L 1 May 1 Apr 1 May 1 Jun 1 Jul 1 Aug 1 Sep 1 Oct 1 Nv 1 Dec Fig. 2 Daily rainfall and agricultural activities at the farm. PHYSICAL CHARACTERISTICS OF THE FARM SOILS The surface sil at the bserved farm cnsists f three kinds f vlcanic sil cntaining plenty f micr-rganisms and misture. T evaluate the hydraulic parameters such as hydraulic cnductivity, prsity and the unsaturated seepage characteristics, experimental measurements using undisturbed sil samples and in situ infiltratin tests were carried ut (Sugi & Okabayashi, 1994). Unsaturated seepage characteristics were expressed by the parameters f the van Genuchten equatin (van Genuchten, 1980) as the fllwing equatins:

3 In situ measurements and numerical simulatin f nitrate leaching frm farm sil 497 NO3-N cncentratin (mg I" ) CO E» 3?i50-2> tt q t cx> 28 Apr 4 Jun 21 Jul 11 Sep O Measur. (West) O Measur. (fast) 27 Oct Fig. 3 Vertical distributin f NO3-N cncentratin in the sil at the west side and the east side f the farm. s. = 1/1+ am kjk. l-(l-sl"")'" m =1 1/n (1) 5, = (0-e r )/(0,-e,) where \\i is capillary suctin head, k u is unsaturated hydraulic cnductivity, k s is saturated hydraulic cnductivity, 0 is vlumetric water cntent, 9 S is saturated water cntent apprximated as the value f prsity and 9,. is residual water cntent. Table 2 shws the parameters identified frm their measurements r tests. Table 2 Unsaturated seepage parameters. And sil Bra sil Silt k s (cm h"') 1.12 x 10" x x 10" 5 e, Q r a n NUMERICAL SIMULATION OF NO3-N LEACHING T understand NO3-N perclatin in the farm, ne-dimensinal unsaturated seepage flw f rainwater was simulated first. The gverning equatin fr the unsaturated seepage flw is expressed by: C W ( V )cv / dt = V(k u ( V ) v> - k u i») (2) where C w is specific misture capacity. As the bundary cnditin, the bserved daily rainfall was applied at the sil surface and the grundwater table was assumed t be fixed at 5 m in depth. Evaptranspiratin prcesses were nt cnsidered.

4 498 N. Yamashita & S. Sugi The leaching prcesses f NO3-N were simulated by NITRAT, which is ne f the useful numerical mdels fr simulating multispecies transprt in saturated grundwater flw (Kinzelbach & Schâfer, 1991). NITRAT was applied t the individual field because the vlumetric water cntent f the surface sil f the farm can be apprximated as saturated due t the weather characteristic f the study area. This mdel describes hetertrphic prcesses, which take int accunt the interactin f xygen, nitrate, rganic carbn and bacteria. The gverning equatins are: dno x J8t = -V(LWC\ J + V(DVN0 3 J - (3(/V6\ w - N0 3M ) (3) ac\ w / dt = -v(r/ 2>w ) + v(z5vc\ w ) - p( 2>w - 2M ) (4) 9C rg: Jdt = -V(UC rg J + V(DVC rg J-$(C rgtw -C rgm ) (5) dn0 3M I dt = - y dm I Y dm -y a J Y aer + P(/VC\ W - N0 3M ) (6) d0 2M I dt = -y aer I Y aer + P(C\ W - 0 2M ) (7) Crgfiw I dt = y dm IY den y aer IY aer + f me y dec + $(C rgw ~C r g b i ) (8) d{x aer + X ien )dt = y mr + y im - y dec (9) where NO3, O2, C rg and X are the cncentratins f nitrate-nitrgen, xygen, rganic carbn and bacteria, respectively; the subscripts w, bi, aer and den indicate pre water, biphase, aerbic and denitrifying, respectively; U is the seepage velcity, D (= ajj) is the dispersin cefficient, ai is the dispersin cnstant, p is the exchange cefficient between pre water and biphase, Y is the yield cefficient, / is utilizable prtin f dead bacteria, and y is the grwth rate f bacteria which is als expressed by the fllwing equatins: Y =\x aer n-f(0 )1 ^ X (10) I aer rimil- V^2 Ji, a e r n ^ r rr aer, Ar/-> \ r rr aer, s\ \ "er ( K c + C rg ) i K NO, + N 3 ) ( K ) y = M F(Q 1 ^ r g ^ 3 V (H) Idea Pmax V^2J, den, r 1 \ linden, 7,770 \ den ( K e rg ) ( NOj ry(j 3 ) 1dec=KX aer +X d J (12) where p m a x is maximum grwth rate f bacteria, Kj is half-velcity cncentratin fr species i, A. is the first-rder decay cefficient, F(02) is xygen dependent weight. The initial cncentratins f each species and the bundary cnditin f rganic carbn and xygen at the grund surface are set as shwn in Table 3 and the values f bilgical parameters used in NITRAT are als set as shwn in Table 4. These parameters were identified frm the results f clumn experiments using And sils (Waner & Gujer, 1984; Sugi, 1997). In the calculatin f NO3-N leaching, the mean value f seepage velcities calculated every 10 days at five distributed intervals (the depths 35, 35, 50, 90 and 90 cm, respectively) were applied t the cnvectin term f the gverning equatins. The bundary cnditin f NO3-N at the sil surface must cincide with the actual applicatin dates and the quantities applied. In the And sil develped at the tp f the

5 In situ measurements and numerical simulatin f nitrate leaching frm farm sil 499 Table 3 Initial and bundary values applied t NITRAT. NOj Q 2 Cr g Initial values (mg l" 1 ) Bundary values at the grund surface (mg I" 1 ) Table 4 Bilgical parameters used in NITRAT. Parameters Values Parameters Values Parameters Values P 10.0 X(mgr') 0.3 K? (mgf 1 ) 0.76 Yden 0.4 a L (m) 0.01 K * ( m g f 1 ) 0.5 Y * aer 0.09 K'" (mgl"') NO, fuse 0.9 K% t ( m S r ') 7.0 p*" x (mgl"') 0.3 A/day 1 ) farm, the nitrgen in NH 4 -N and N0 2 --N are xidized quickly by bilgical nitrificatin and are changed t NO3-N in the sil; the NO3-N is then leached under the grund with the seepage f rainwater (Sugi & Imamura, 1991). Therefre, bundary cncentratins at the sil surface were applied as fllws: the rainwater disslves barnyard manure NO3-N at a cncentratin f 300 mg l" 1, and the chemical fertilizer at 600 mg l" 1, and the frage crps absrb 50% f NO3-N after seeding. Based n these assumptins, the disslutin perids f barnyard manure and fertilizer are evaluated as shwn in Table 5. Table 5 Disslutin perids f manure and fertilizer, and accumulated rainfall. Manuring r fertilizing date 1 March April May 1998 Disslutin perids (day) 70 (up t 5 May) 15 (up t 12 May) 20 (up t 9 June) Accumulated rainfall (mm) SIMULATED RESULTS Figure 4 shws the measured and simulated vertical distributin f NO3-N cncentratin in the farm. Althugh the unsaturated perclatin f NO3-N was simulated apprximately by NITRAT when applying the mean value f the unsaturated seepage velcity, the simulated results agreed well with the measured results. Accrding t the simulated results, tw cncentratin peaks are generated n 28 April after manuring n 3 March. Rainfall ccurring between the manuring date and the middle f April leads t the lwer peak, and heavy rainfall at the end f April leads t the upper peak (Fig. 2). The number f cncentratin peaks changes frm three n 4 June t ne n 27 Octber with leaching frm sil belw grund level. Therefre, it is clear that sme f the cncentratin peaks are generated in the surface zne after manuring cmbined with the influence f a rainfall event, and that these peaks are balanced by leaching belw the surface. It is imprtant t nte that ne f the causes f grundwater cntaminatin is ver applicatin f barnyard manure in the study area. An attempt was made t estimate the

6 500 N. Yamashita & S. Sugi NO3-N cncentratin (mg I" 1 ) Measur. (West) 0 Measur. (East) Sim. 28 Apr 4 Jun 21 Ju] 11 Sep 27 Oct Fig. 4 Cmparisn f simulated results f N0 3-N cncentratin with measured results at the farm f=t H i 80 8 & 40 H O Actual quantity Half quantity f barnyard m anure 0 1 Mar 20 Apr 9 Jun 29 Jul 17 Sep 6 Nv 26 Dec Fig. 5 Simulated NO3-N cncentratin at 100 cm depth in half quantity manuring case. effect f reduced applicatins f barnyard manure by means f simulatin. Figure 5 shws a result f the daily variatin f NO3-N cncentratin at a depth f 100 cm. The slid line shws NO3-N cncentratin in the case f the actual quantity f the barnyard manure and the fertilizer. The circle marks shw it in the case f the same surces but with nly half the quantity f barnyard manure. The results fr the reduced applicatin case shw a lwer peak cmpared t the actual case. The lwer peak is cnsidered t be caused by reductin f the disslutin perids f manure and by separatin t disslutin perids f fertilizer. The result indicates that NO3-N cncentratin in the sil can be reduced by cnsidering apprpriate dates and quantities fr manuring. REFERENCES Gillham, R. W. & Webber, L. R. (1969) Nitrgen cntaminatin f grundwater by barnyard leachates. J. Wat. Pllut. Cntrl Fed. 41(10), Kinzelbach, W. & Schâfer, W. (1991) Numerical mdeling f natural and enhanced denitrificatin prcesses in aquifers. Wat. Resur. Res. 27(6),

7 In situ measurements and numerical simulatin f nitrate leaching frm farm sil 501 Sugi, S. & Imamura, T. (1991) Disslutin f fertilizer in And sils. J. Grundwat. Hydrl. 33(3), Sugi, S. & Okabayashi, T. (1994) Unsaturated permeability f Shirasu and its in-situ measuring methds. J. Hydraul. Castal & Envirn. Engng JSCE 503/11-29, 39^17. Sugi, S. (1997) Experiments and numerical simulatins f nitrate transprt in And sil. In: Prc. Twenty-seventh Cng. ffahr, Theme C, van Genuchten, M. Th. (1980) A clsed-frm equatin fr predicting the hydraulic cnductivity f unsaturated sils. Sil Sci. Sc. Am. J. 44(5), Waner, O. & Gujer, W. (1984) Cmpetitin in bifilms. Wat. Sci. Technl. 17, Yamashita, N. & Sugi, S. (1999) Mvement f applied nitrgen in frage crps field. In: Prc. Annual Meeting Western Div. JSCE,

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