1

The role of the unsaturated zone in determining nitrate leaching to

groundwater

micòl MATROCICCO1,2), nicolò COLOMBANI1,2), enzo SALEMI1), fabio VINCENZI3), giuseppe

CASTALDELLI 2,3)

1) Department of Earth Sciences, University of Ferrara, Via Saragat, 1, 44100 Ferrara, Italy 2) LT Terra&Acqua Tech, HTN Emilia-Romagna, Via L. Borsari, 46, 44100 Ferrara, Italy 3) Department of Biology and Evolution, University of Ferrara, Via L. Borsari, 46, 44100 Ferrara, Italy

Abstract

In order to identify the dominant processes affecting nitrate leaching in the Po River Delta area, field tests were

performed to determine the fate and transport of nitrogen species. Nitrogen (urea) was applied at a rate of 300

kg-N/ha/y, in both a sandy and a silty loamy sites cultivated with maize; the sandy soil was amended with

chicken manure (700 kg/ha), while the silty loamy soil never received chicken manure amendment. Each field

site was equipped with soil moisture probes, suction cups and piezometers to quantify the presence of nitrogen

and carbon dissolved species in the subsurface. Nitrate leaching was observed in the silty loamy soil, while in the

sandy soil the elevated dissolved organic matter, resulting from chicken manure decomposition, prevented the

nitrate migration towards the aquifer. Results highlight the reliability of increasing the labile organic matter in

the more permeable and intrinsically vulnerable sandy soil to prevent nitrate leaking.

1. Introduction

Nitrate (NO3-) is a pervasive inorganic pollutant often found in shallow aquifers (Galloway et al.,

2008; Rivett et al., 2008). NO3- contamination is due to agricultural fertilization and other sources, like

industrial discharges and municipal sewer systems (Wakida & Lerner, 2005). NO3- concentrations are

frequently found spatially and temporally variable in aquifers (Böhlke et al., 2002; Thayalakumaran et

al., 2008), this is usually related to variations in groundwater flow direction and nitrate attenuation

(Tesoriero et al., 2000; Almasri & Kaluarachchi, 2007). In Italy, the Po River valley is the largest and

more intensively farmed alluvial plain and is heavily impacted by NO3- groundwater contamination

(Mastrocicco et al., 2010a; Onorati et al., 2006; Cinnirella et al., 2005) and surface water

eutrophication (Provini et al., 1992; Palmieri et al., 2005). However, in agricultural practices, the

types of soils and soil tillage, different crops and irrigation techniques and different nitrogen

2

fertilizers, form a variety of terms emphasizing site specificity of nitrogen load and subsequent

denitrification (Seitzinger et al., 2006). A generally well understood and quantified process of nitrogen

attenuation from surface and groundwater systems is the heterotrophic denitrification, this process

uses NO3- as electron acceptor and a carbon source as electron donor, producing nitrogen gases

(Coyne, 2008; Schipper et al., 2008).

The purpose of this research was to investigate the fluxes of NO3- from the top soil to the groundwater

in cultivated plots and to determine if the addition of chicken manure from organic farming is a viable

alternative to diminish the NO3- concentration in shallow groundwater bodies. This was tested in two

different sites with the same fertilization rates but with different soil textures, sandy and silty loamy.

In the sandy soil, chicken manure was employed to increase its low intrinsic fertility by augmenting

the soil labile organic matter (Whitmore, 2007).

2. Materials and methods

2.1. Field sites

The entire Po delta area is an intensively farmed region due to its flat topography and abundance of

surface water for irrigation; the primary agricultural land use is maize cropping. In the study area,

located in Ferrara province (Italy) at an altitude ranging from 5 to -3 m above sea level (a.s.l.), two

sites (named CCR and MON) were selected to monitor the water and nitrogen transport in the

unsaturated/saturated zone. Both the sites are cultivated under a rotation of cereals, mainly maize and

wheat, using urea as nitrogen fertilizer at an average rate of 300 kg-N/ha/y. The surface area of the

plot in each site was 1 ha, its slope was less than 0.5% (and mostly less than 0.05%). For this reason, it

was assumed surface runoff has been minimal and water movement in the unsaturated zone has been

dominantly vertical. Meteorological stations recording rainfall, wind speed, solar radiation,

temperature and humidity are located from 0.5 to 5 km far from the field sites. Data are available on-

line from meteorological regional service (www.dexter.it) and from local web service

(www.meteoveneto.com). A rain gauge was installed in each site to record daily rainfall. The linear

correlation of daily data between the rain gauges and the nearest regional weather stations (R2>0.9 for

all sites), consented to extend the data measured by the on line weather stations to the field sites.

Table 1: average meteorological parameters for the two field sites during the two monitored years.

Cumulative Rainfall

(mm)

Average Air

Temp. (°C)

Solar Radiation

(MJ/m2/d)

Humidity

(%)

Wind speed

(Km/d)

CCR 1813 13.2 1.60E-04 76 5.5

MON 999 13.6 1.66E-04 70 8.6

3

To preserve natural conditions no irrigation was applied. As shown in Table 1 the average

meteorological parameters recorded in the two sites are typical of coastal plain environments, with a

sub-coastal climate characterized by cold winters and warm summers, with moderate precipitations,

elevated humidity, low wind speed, moderate daily and seasonal temperature variations.

The predominant soil textures in Ferrara province are silty loam and silty clay (68% of the territory),

while sandy soils are less common (11% of the territory). The CCR soils are in general moderately

alkaline, with the upper horizons characterized by silty clay loam texture and moderate carbonates

content; the lower horizons exhibit silty loam texture and are highly calcareous (Tab. 2). Briefly, the

hydrogeological units present in the CCR site are the unconfined aquifer composed of recent fluvial

sandy deposits with clay and silt lenses, from 0 to around 4 m below ground level (b.g.l.), and the

underlying aquiclude constituted of fluvial clay and silty sediments, from 4 to almost 14 m b.g.l..

The representative MON soil profile shows upper horizons of approximately 40-60 cm thickness

characterized by fine sand texture, with moderate carbonates content and slightly alkaline pH; while

the lower horizons exhibit alkaline pH and medium sand texture (Tab. 2). The hydrogeological units

present in the MON site are the unconfined aquifer composed of coastal plain medium and fine sandy

deposits, from 0 to around 12 m (b.g.l.), and the underlying aquiclude constituted of prodelta silt and

clay sediments, from 12 to almost 15 m b.g.l. (Stefani & Vincenzi, 2005).

Table 2: Soil characterization for the two field sites

Pedological classification Sedimentological environment Textural classification

CCR Haplic Calcisols fluvial silty loam

MON Calcaric Arenosols coastal plain sand

2.2. Analytical and field methods

To better define the site stratigraphy triplicates core logs were drilled manually with an Ejielkamp

Agrisearch auger equipment down to 2 m (b.g.l.). The soil stratification was divided in two distinct

layers: the upper one stressed by tillage, roots growth and weathering and the lower undisturbed one

(Tab. 3). In CCR site the upper layer was 0.75 m thick and in MON was 0.65 m, while the lower layer

extended until 2 m b.g.l. at both sites. From collected core samples at 0.25, 0.50, 0.75, 1 and 2 m

b.g.l., particle size curves were obtained using a settling tube for the sandy fraction and an X-ray

Micromeritics Sedigraph 5100 for the finer one. Organic matter content was measured by loss of

ignition method (Tiessen & Moir, 1993), while bulk density was determined gravimetrically.

4

Table 3: average grain size distribution, bulk density and organic matter content, measured for the

upper and lower layers.

Parameter CCR

Upper Layer

CCR

Lower Layer

MON

Upper Layer

MON

Lower Layer

Grain size (%)

Sand

Silt

Clay

Bulk density (g/cm3)

Organic matter (%)

7.7

63.2

29.1

1.5

2.3

23.9

58.1

18.0

1.6

1.3

95.6

3.0

1.4

1.4

2.0

98.1

1.9

0.0

1.7

1.1

Two arrays of Watermark soil moisture probes were vertically inserted into augered holes, at the same

depths of 0.25, 0.50, 0.75 and 1 m b.g.l. in each field site. Watermark soil moisture probes were used

to monitor the soil water potential (measurement range 0-250 cbar). A copper-constantan

thermocouple was inserted adjacent to each soil moisture probe to compensate for soil temperature.

Standard Irrometer tensiometers (measurement range 0-80 cbar) were installed at 0.25 and 0.50 m

depths to monitor and correct any deviance of soil moisture probes readings.

A series of nested piezometer (2.5 cm inner diameter) screened from 1.5 to 4 m a.s.l., were installed

near the soil moisture arrays to monitor the level and quality of groundwater. Monitoring started on 27

March 2008 and is still going on. LTC M10 Levelogger Solinst dataloggers were placed in

piezometers to monitor hourly groundwater level, electrical conductivity and temperature. All the

piezometers were sampled at variable intervals, via low flow purging, for major ions and TOC/TIC

determination. Two arrays of soil solution suction samplers were installed at 0.25, 0.50, 0.75 and 1 m

b.g.l. in each site to analyze soil water in the unsaturated zone. In addition to suction sampler, the

unsaturated zone was sampled every four months by means of auger coring (from 0 to 2 m b.g.l.), and

sediments were analyzed for major anions and cations. Unsaturated zone sediment analysis consisted

of a batch with a sediment/water ratio of 1:10, using 10 g of air dried sample dispersed in 100 ml of

Milly-Q water (Millipore, US). A biological inhibitor (1 g/l phenylmercuric acetate) was added to

prevent microbial activity and the solution was stirred for 1 hour and then allowed to stand for one

day. The insoluble residue was removed by filtration and analyzed for major cations and anions. In-

well parameters were determined with the HANNA Multi 340i instrument which includes a HIcell-31

pH combined electrode with a built-in temperature sensor for pH measurements, a CellOx 325

galvanic oxygen sensor for DO measurements, a combined AgCl-Pt electrode for Eh measurement

and a HIcell-21 electrode conductivity cell for EC measurements. The major cations, anions and

oxianions (acetate and formate) were determined with isocratic dual pump ion chromatography ICS-

1000 Dionex, equipped with an AS9-HC 4 x 250 mm high capacity column and an ASRS-ULTRA

5

4mm self-suppressor for anions, and a CS12A 4 x 250 mm high capacity column and a CSRS-ULTRA

4mm self-suppressor for cations. Samples were filtered through 0.22 µm Dionex vial caps. An AS-40

Dionex auto-sampler was employed to run the analyses, Quality Control (QC) samples were run every

10 samples. The standard deviation for all QC samples run was better than 4%. Charge balance errors

in all analyses were less than 5% and predominantly less than 3%. Total organic carbon (TOC) and

total inorganic carbon (TIC) were determined with a carbon analyzer (Carbon Analyzer Shimadzu

TOC-V-CSM) after acidification with one drop of 2 M HCl to remove dissolved carbonate.

3. Results and discussion

3.1 Unsaturated zone monitoring

The matric potential measured at different depth in CCR (Fig. 1) shows that during the autumn/winter

seasons the upper and the lower horizons are near the saturation state. This implies that recharge is

taking place especially during the late winter season, where the saturation state is reached in all the

measuring point concomitantly. On the contrary, from the sowing to the harvest of maize (May to

September 2009) the soil became dry in the upper horizon since the evapotranspiration was elevated

(Mastrocicco et al. 2010b).

t For crop rotation needs, in May 2010 beetroots were sowed instead of maize at CCR site and the soil

became dry also in the lower horizon, since the rooting system of the beetroot is deeper than in maize

(Christiansen et al., 2006). In addition Figure 1 shows a clear temporal shift during the wetting cycle,

from the sensor located at 0.25 m b.g.l. and the one located at 0.5 m b.g.l.; this is due to the low

permeability of these soils, which do not allow fast vertical transfer of water.

6

Figure 1: rainfall and matric potential (in cbar) recorded at CCR site during the monitoring period at

different depth 0.25, 0.5, 0.75 and 1.0 m b.g.l.

Figure 2 depicts the matric potential measured at different depths in MON, here the upper soil did not

reach the values recorded in CCR because the water table was on average at 0.8 m b.g.l.. This

condition provided a continuous source of water for the maize rooting system, which did not

necessitate of irrigation during the cropping season. As a consequence, the lower horizon was always

saturated with groundwater and the elevated permeability of these sandy soils allowed fast travel times

of recharge water towards the shallow unconfined aquifer.

It is also noticeable that during the summer 2009 the soil became dry to 0.5 m b.g.l., while during the

summer 2010 only the first 0.25 m were unsaturated. This was due to the water table that was slightly

higher during summer 2010 (see Figure 6 in section 3.3).

7

Figure 2: rainfall and matric potential (in cbar) recorded at MON site during the monitoring period at

different depth 0.25, 0.5, 0.75 and 1.0 m b.g.l.

3.2 Unsaturated/saturated zone profiles

Since the most intense recharge occurs during spring time, Figure 3 shows a series of NO3- profile

collected in CCR and in MON sites in April 2009 and 2010 before the fertilization. In CCR the NO3-

concentration observed in April 2009 showed a peak at 3 m a.s.l. in correspondence of the water table,

while in the saturated zone NO3- decreased rapidly to concentrations below 50 mg/l.

In April 2010 the observed NO3- profile showed very high concentration with a maximum peak below

the water table. The elevated NO3- concentration was due to a combination of factors: (i) the water

table was higher than in 2009, (ii) the temperature was lower than in 2009 and (iii) precipitation was

less intense.

8

Figure 3: NO3- depth profiles at CCR (on the left) and at MON (on the right) collected in April 2009

and April 2010 (concentrations measured in solids corrected for bulk density and water content; error

bars show the standard deviation of three replicates)

The higher water table suggests that more NO3- trapped in the vadose zone can be dissolved in

groundwater, the lower temperature may decrease denitrification rate and lower rainfall contributes to

concentrate NO3-. The same trend is visible for the MON site, although NO3

- concentrations were

about five times lower and the profile appears quite different from CCR. In fact, NO3- was found only

in the upper part of the profile, while below 1 m a.s.l. NO3- was always below detection limits. Nitrate

disappearance below 1 m a.s.l. was attributed to denitrification supported by the addition of chicken

manure, in April 2008, which provided labile organic matter used as an electron donor. The process is

microbially catalyzed and can be written as an overall reaction:

5CH2O + 4NO3- → 4HCO3

- + CO2 + 2N2 + 3H2O

The process involves also nitrite as intermediate compounds that are transiently produced and then

reduced to nitrogen gas. When denitrification is not limited by organic substrate, nitrite remains at low

concentration. Otherwise, when organic substrates become limiting nitrite tends to accumulate (Nair et

al., 2007; Israel et al., 2009).; In fact, in MON site nitrite remained at very low level, always below 1

mg/l, while in CCR site concentrations up to 100 mg/l were occasionally recorded in suction cups.

This provided an evidence of incomplete denitrification due to a lack of organic substrate. Ammonium

was always observed below 2 mg/l and thus not shown.

In accordance, Figure 4 shows the TOC, acetate and TIC in both sites collected in April 2009 and

April 2010. The TOC observed in CCR site was always lower than the TOC in MON site, this can be

directly linked to chicken manure addition at MON site. Acetate concentration followed the same

trend of TOC in both sites: in fact TOC decreased with depth and also acetate, but the latter became

exhausted at shallower depths since this organic acid is quickly available as carbon source

9

(Mastrocicco et al., 2011). Acetate is one of the most reactive organic acids, produced as intermediate

during organic matter degradation and thus is a good proxy to evaluate the reactivity of organic matter

(Strobel, 2001). The elevated abundance of TOC after the addition of chicken manure in MON is

another evidence of the excess of electron donors compared to nitrate. This led to a full nitrate

reduction within the first meter of soil. On the contrary, at CCR site the TOC and acetate

concentrations were not sufficient to support denitrification, leading to accumulation of nitrite. This

conceptual model was also supported by the measured dissolved oxygen at MON and CCR sites in the

piezometers: at MON oxygen was always below detection limits, while in CCR oxygen was present

between 3 to 5 mg/l. Finally, the TIC vertical distribution supports the postulated denitrification

reaction, because in MON the TIC concentration was elevated in the vadose zone, where the NO3- was

consumed. While, in CCR the TIC increased towards the bottom of the aquifer, simply following the

accumulation trend of total dissolved species (not shown).

10

Figure 4: TOC, acetate and TIC depth profiles at CCR (on the left) and at MON (on the right)

collected in April 2009 and April 2010 (concentrations measured in solids corrected for bulk density

and water content; error bars show the standard deviation of three replicates)

3.3 Saturated zone monitoring

Figure 5 shows a variable groundwater table with large seasonal variations in the loamy soil , where

the groundwater flow is linked with canals level. In particular, the sharp peak recorded the 21/06/2010

11

was due to a flood event that increased the level of the nearby canal of 2.5 m, but the soil moisture

sensor placed at 1m b.g.l. was not in saturation condition (Figure 1). This proves that the peak was due

to groundwater fluctuation induced by the canal and not by recharge.

Figure 5: groundwater level fluctuations of NO3- and EC trends in groundwater at CCR (loamy soil)

throughout the monitoring period.

At the CCR site nitrate mass transfer to the unconfined aquifer was slow as shown in Figure 5 and

concentrated at the end of the winter season, when the water table rose and brought in solution the

available NO3-. For comparison with NO3

- in Figure 5 is plotted also the EC, but is evident that there is

not a direct relation between these two parameters. EC was generally increasing when the water table

was rising, since the latter dissolved the salts accumulated in the vadose zone. While during the

summer seasons EC decreased because groundwater was replaced by canal water that had a lower EC.

Figure 6: groundwater level fluctuations of NO3- and EC trends in groundwater at MON site (sandy

soil) throughout the monitoring period.

12

Figure 6 shows that in the MON site groundwater fluctuations were less pronounced than in CCR,

since MON site is located near the coast and the water table fluctuations are smoothed. At the MON

site, despite the fast transfer of mass induced by the elevated permeability of the soil, NO3-

concentration was very low. In fact, NO3- concentration never exceeded 2 mg/l confirming that the

denitrification process efficiently removed all the NO3- as explained above (paragraph 3.2). The EC

monitoring was less continuous due to malfunction of the EC probe, thus only the EC measured during

sampling campaigns is shown. In general the EC seems to be related to the rainfall, exhibiting

decreasing values after prolonged rainy periods although is not possible to infer clear trends with these

sparse data. Like for the CCR site, the EC trend can not be directly related to NO3- concentration in

groundwater.

4. Conclusions

The proposed approach has evidenced that in the sandy soil, the release of organic substrates from the

mineralization of manure was sufficient to prevent nitrate leaking. Therefore this study emphasizes the

importance of considering the role of labile organic matter in buffering NO3- excess via denitrification

reactions. In particular, the augmentation of the soil labile organic matter with chicken manure,

provided a source of labile organic acids, like acetate, used as substrates for the denitrification process.

On the other hand, in the unamended silty loamy soil the chronic deficiency of labile organic matter,

could not prevent nitrogen leaking towards the shallow unconfined aquifer.

According to the sustainable agricultural practices recommended by the Water Framework Directive

(2000/60 EU) to prevent groundwater pollution, this approach has demonstrated that even in the

intrinsically more vulnerable area (the sandy soil), a relatively low but constant release of organic

substrates was sufficient to prevent nitrate leaking. Nevertheless, further research should pay attention

to the kind of manure used and to the farming practices, organic or industrial. In fact, this last term

may influence organic matter transformation kinetics and nitrogen mineralization rates, because of the

presence of hormones, antibiotics and other undesired chemicals that could impact the groundwater

quality and could interfere with bacterial activities. Although in literature there is an increasing

interest on using 15N and 18O nitrate isotopes to distinguish the origin of nitrate, and numerical

modelling of nitrate leaching to quantify the mass transport; the present study was done on relatively

simple sites, where depth dependent monitoring allowed to distinguish between different flow and

reactions pathways. Nevertheless, the isotopic and numerical modelling approaches are key tools to

track the fate of nitrogen species in the subsurface.

Acknoledgments

13

Dr. Umberto Tessari and Dott. Sandro Bolognesi are acknowledged for the technical and scientific

support. The Emilia–Romagna ARPA SIMC is acknowledge for the meteorological data. The work

presented in this paper was made possible and financially supported by PARC-AGRI under “Contratto

di Programma (Delib.CIPE n.202)” and by the Province of Ferrara within the EU-Water Project of the

South-East Europe Program.

References

Almasri M N, Kaluarachchi J J (2007) Modeling nitrate contamination of groundwater in agricultural

watersheds. J of Hydrol 343: 211-229

Böhlke J K, Wanty R, Tuttle M, Delin G, Landon M (2002) Denitrification in the recharge area and

discharge area of a transient agricultural nitrate plume in a glacial outwash sand aquifer, Minnesota.

Wat Resour Res 38 (7): 10.1–10.26

Christiansen J S, Thorup-Kristensen K and Kristensen H L (2006) Root development of beetroot,

sweet corn and celeriac, and soil N content after incorporation of green manure. J Horticult Sci &

Biotech 81 (5): 831–838

Cinnirella S, Buttafuoco G, Pirronea N (2005) Stochastic analysis to assess the spatial distribution of

groundwater NO3- concentrations in the Po catchment (Italy). Environmental Pollution 133: 569–580

Coyne M S (2008) Biological denitrification. In: Schepers J S, Raun W (eds) Nitrogen in Agricultural

Systems, ASA-CSSSA-SSSA Agronomy Monograph 49, Madison, WI, pp. 197–249

Galloway J N, Townsend A R, Erisman J W, Bekunda M, Cai Z, Freney J R, Martinelli L A,

Seitzinger S P, Sutton M A (2008) Transformation of the nitrogen cycle: recent trends, questions, and

potential solutions. Science 320: 889-892

Giuliano G (1995) Ground water in the Po basin: some problems relating to its use and protection. Sci

Total Environ 171: 17–27

Israel S, Engelbrecht P, Tredoux G, Fey M V (2009) In situ batch denitrification of nitrate-rich

groundwater using sawdust as a carbon source—Marydale, South Africa. Water Air & Soil Poll 204:

177-194

14

Mastrocicco M, Colombani N, Castaldelli G, Jovanovic N (2010a) Monitoring and modeling nitrate

persistence in a shallow aquifer. Water Air & Soil Poll 217: 83-93

Mastrocicco M, Colombani N, Salemi E, Castaldelli G (2010b) Numerical assessment of effective

evapotranspiration from maize plots to estimate groundwater recharge in lowlands. Agricult Wat

Manag 97(9): 1389-1398

Mastrocicco M, Colombani N, Salemi E, Castaldelli G (2011) Reactive Modeling of Denitrification in

Soils with Natural and Depleted Organic Matter. Water Air & Soil Poll doi: 10.1007/s11270-011-0817-6

Onorati G, Di Meo T, Bussettini M, Fabiani C, Farrace M G, Fava A, Ferronato A, Mion F, Marchetti

G, Martinelli A and Mazzoni M (2006) Groundwater quality monitoring in Italy for the

implementation of the EU water framework directive. Phys and Chem of the Earth 31: 1004–1014

Nair R R, Dhamole P B, Lele S S, D’Souza S F (2007) Biological denitrification of high strength

nitrate waste using preadapted denitrifying sludge. Chemosphere 67: 1612-1617

Palmeri L, Bendoricchio G and Artioli Y (2005) Modelling nutrient emissions from river systems and

loads to the coastal zone: Po River case study, Italy. Ecolog Modelling 184: 37–53

Provini A, Crosa G, Marchetti R (1992) Nutrient export from the Po and Adige river basin over the

last 20 years. Sci Total Environ Suppl: 291-313

Rivett M O, Buss S R, Morgan P, Smith J W N, Bemment C D (2008) Nitrate attenuation in

groundwater: A review of biogeochemical controlling processes. Water Res 42: 421-4232

Seitzinger S, Harrison J A, Böhlke J K, Bouwman A F, Lowrance R, Peterson B, Tobias C, and Van

Drecht G (2006) Denitrification across landscapes and waterscapes: A synthesis. Ecolog Appl 16(6):

2064-2090

Schipper L A, Robertson W D, Gold A J, Jaynes, D B, Cameron S. C (2010) Denitrifying bioreactors-

An approach for reducing nitrate loads to receiving waters. Ecolog Eng

doi:10.1016/j.ecoleng.2010.04.008

15

Stefani M, Vincenzi S (2005) The interplay of eustasy, climate and human activity in the late

Quaternary depositional evolution and sedimentary architecture of the Po Delta system. Marine Geol

222–223: 19–48.

Strobel B W (2001) Influence of vegetation on low-molecular-weight carboxylic acids in soil

solution—a review. Geoderma 99: 169-198

Tesoriero A J, Liebscher H, Cox S E (2000) Mechanism and rate of denitrification in an agricultural

watershed: Electron and mass balance along groundwater flow paths. Wat Resour Res 36(6): 1545–

1559

Thayalakumaran T, Bristow K L, Charlesworth P B, Fass T (2008) Geochemical conditions in

groundwater systems: Implications for the attenuation of agricultural nitrate. Agricult Wat Manag 95:

103-115

Tiessen H, Moir J O (1993) Total and organic carbon. In: Soil Sampling and Methods of Analysis,

M.E. Carter, (Ed) Lewis Publishers: Ann Arbor, MI, pp. 187-211

Youngs E G (1995) Developments in the physics of infiltration. Soil Sci Soc Am J 59(2): 307–313

Wakida F T, Lerner D N (2005) Non-agricultural sources of groundwater nitrate: A review and case

study. Water Res 39(1): 3–16

Whitmore A (2007) Determination of the mineralization of nitrogen from composted chicken manure

as affected by temperature. Nutr Cycl Agroecosyt 77(3): 225-232