Thomas Harter

Shallow groundwater quality on dairy farms with irrigated forage crops

Californias dairies are the largest confined animal industry in the state. A major portion of these dairies, which have an average herd size of nearly 1000 animal units, are located in low-relief valleys and basins. Large amounts of liquid manure are generated and stored in these dairies. In the semi-arid climate, liquid manure is frequently applied via flood or furrow irrigation to forage crops that are grown almost year-round. Little is known about the impact of manure management practices on water quality of the extensive alluvial aquifers underlying these basins. The objective of this work is to assess nitrate and salt leaching to shallow groundwater in a relatively vulnerable hydrogeologic region and to quantify the impact from individual sources on dairies. The complex array of potential point and nonpoint sources was divided into three major source areas representing farm management units: (1) manure water lagoons (ponds); (2) feedlot or exercise yard, dry manure, and feed storage areas (corrals); and (3) manure irrigated forage fields (fields). An extensive shallow groundwater-monitoring network (44 wells) was installed in five representative dairy operations in the northeastern San Joaquin Valley, CA. Water quality (electrical conductivity, nitrate-nitrogen, total Kjehldahl nitrogen) was observed over a 4-year period. Nitrate-N, reduced nitrogen and electrical conductivity (EC, salinity) were subject to large spatial and temporal variability. The range of observed nitrate-N and salinity levels was similar on all five dairies. Average shallow groundwater nitrate-N concentrations within the dairies were 64 mg/l compared to 24 mg/l in shallow wells immediately upgradient of these dairies. Average EC levels were 1.9 mS/cm within the dairies and 0.8 mS/cm immediately upgradient. Within the dairies, nitrate-N levels did not significantly vary across dairy management units. However, EC levels were significantly higher in corral and pond areas (2.3 mS/cm) than in field areas (1.6 mS/cm) indicating leaching from those management units. Pond leaching was further inferred from the presence of reduced nitrogen in three of four wells located immediately downgradient of pond berms. The estimated minimum average annual groundwater nitrate-N and salt loading from manure-treated forage fields were 280 and 4300 kg/ha, respectively. Leaching rates for ponds are estimated to be on the order of 0.8 m/year, at least locally. Since manure-treated fields represent by far the largest land area of the dairy, proper nutrient management will be a key to protecting groundwater quality in dairy regions overlying alluvial aquifers.

Use and Environmental Occurrence of Antibiotics in Freestall Dairy Farms with Manured Forage Fields

Environmental releases of antibiotics from concentrated animal feeding operations (CAFOs) are of increasing regulatory concern. This study investigates the use and occurrence of antibiotics in dairy CAFOs and their potential transport into first-encountered groundwater. On two dairies we conducted four seasonal sampling campaigns, each across 13 animal production and waste management systems and associated environmental pathways: application to animals, excretion to surfaces, manure collection systems, soils, and shallow groundwater. Concentrations of antibiotics were determined using on line solid phase extraction (OLSPE) and liquid chromatography-tandem mass spectrometry (LC/MS/MS) with electrospray ionization (ESI) for water samples, and accelerated solvent extraction (ASE) LC/MS/MS with ESI for solid samples. A variety of antibiotics were applied at both farms leading to antibiotics excretion of several hundred grams per farm per day. Sulfonamides, tetracyclines, and their epimers/isomers, and lincomycin were most frequently detected. Yet, despite decades of use, antibiotic occurrence appeared constrained to within farm boundaries. The most frequent antibiotic detections were associated with lagoons, hospital pens, and calf hutches. When detected below ground, tetracyclines were mainly found in soils, whereas sulfonamides were found in shallow groundwater reflecting key differences in their physicochemical properties. In manure lagoons, 10 compounds were detected including tetracyclines and trimethoprim. Of these 10, sulfadimethoxine, sulfamethazine, and lincomycin were found in shallow groundwater directly downgradient from the lagoons. Antibiotics were sporadically detected in field surface samples on fields with manure applications, but not in underlying sandy soils. Sulfadimethoxine and sulfamethazine were detected in shallow groundwater near field flood irrigation gates, but at highly attenuated levels.

Transport of Cryptosporidium parvum Oocysts through Vegetated Buffer Strips and Estimated Filtration Efficiency

ABSTRACT Vegetated buffer strips were evaluated for their ability to remove waterborne Cryptosporidium parvum from surface and shallow subsurface flow during simulated rainfall rates of 15 or 40 mm/h for 4 h. Log10 reductions for spiked C. parvum oocysts ranged from 1.0 to 3.1 per m of vegetated buffer, with buffers set at 5 to 20% slope, 85 to 99% fescue cover, soil textures of either silty clay (19:47:34 sand-silt-clay), loam (45:37:18), or sandy loam (70:25:5), and bulk densities of between 0.6 to 1.7 g/cm3. Vegetated buffers constructed with sandy loam or higher soil bulk densities were less effective at removing waterborne C. parvum (1- to 2-log10 reduction/m) compared to buffers constructed with silty clay or loam or at lower bulk densities (2- to 3-log10 reduction/m). The effect of slope on filtration efficiency was conditional on soil texture and soil bulk density. Based on these results, a vegetated buffer strip comprised of similar soils at a slope of ≤20% and a length of ≥3 m should function to remove ≥99.9% of C. parvum oocysts from agricultural runoff generated during events involving mild to moderate precipitation.

Stochastic analysis of solute transport in heterogeneous, variably saturated soils

Statistical moments of solute plumes from small sources in variably saturated, heterogeneous porous media are analyzed by using a newly developed, efficient high-resolution Monte Carlo technique. In agreement with previous theoretical work, it is illustrated that the prediction of such solute plumes is associated with large uncertainties for dimensionless travel times, t′, exceeding 40, particularly predictions of plumes in highly heterogeneous soils (σy2>2). Uncertainty about the travel path of the plume center contributes significantly to overall concentration uncertainty as flux fields become more variable. It is shown that the concentration coefficient of variation at the center of the plume initially increases but stagnates or decreases at later times. For highly heterogeneous soil flux conditions or for the common case of soils with strongly anisotropic conditions, analytical models underestimate transverse spreading of the mean concentration plume at any given time, while overestimating longitudinal spreading. At identical mean plume displacement distances, analytical models underestimate both transverse and longitudinal spreading and overestimate the variance of solute flux (breakthrough curve) by up to a factor 4. As an alternative to the statistical analysis of solute flux, we propose to analyze statistical properties of time associated with peak solute flux and with first exceedance of a given solute flux level.

Assessment of sources and fate of nitrate in shallow groundwater of an agricultural area by using a multi-tracer approach

Nitrate isotopic values are often used as a tool to understand sources of contamination in order to effectively manage groundwater quality. However, recent literature describes that biogeochemical reactions may modify these values. Therefore, data interpretation is difficult and often vague. We provide a discussion on this topic and complement the study using halides as comparative tracers assessing an aquifer underneath a sub-humid to humid region in NE Mexico. Hydrogeological information and stable water isotopes indicate that active groundwater recharge occurs in the 8000km(2) study area under present-day climatic and hydrologic conditions. Nitrate isotopes and halide ratios indicate a diverse mix of nitrate sources and transformations. Nitrate sources include organic waste and wastewater, synthetic fertilizers and soil processes. Animal manure and sewage from septic tanks were the causes of groundwater nitrate pollution within orchards and vegetable agriculture. Dairy activities within a radius of 1,000 m from a sampling point significantly contributed to nitrate pollution. Leachates from septic tanks caused nitrate pollution in residential areas. Soil nitrogen and animal waste were the sources of nitrate in groundwater under shrubland and grassland. Partial denitrification processes helped to attenuate nitrate concentration underneath agricultural lands and grassland, especially during summer months.

Identifying sources of groundwater nitrate contamination in a large alluvial groundwater basin with highly diversified intensive agricultural production

Groundwater quality is a concern in alluvial aquifers underlying agricultural areas worldwide. Nitrate from land applied fertilizers or from animal waste can leach to groundwater and contaminate drinking water resources. The San Joaquin Valley, California, is an example of an agricultural landscape with a large diversity of field, vegetable, tree, nut, and citrus crops, but also confined animal feeding operations (CAFOs, here mostly dairies) that generate, store, and land apply large amounts of liquid manure. As in other such regions around the world, the rural population in the San Joaquin Valley relies almost exclusively on shallow domestic wells (≤150 m deep), of which many have been affected by nitrate. Variability in crops, soil type, and depth to groundwater contribute to large variability in nitrate occurrence across the underlying aquifer system. The role of these factors in controlling groundwater nitrate contamination levels is examined. Two hundred domestic wells were sampled in two sub-regions of the San Joaquin Valley, Stanislaus and Merced (Stan/Mer) and Tulare and Kings (Tul/Kings) Counties. Forty six percent of well water samples in Tul/Kings and 42% of well water samples in Stan/Mer exceeded the MCL for nitrate (10mg/L NO3-N). For statistical analysis of nitrate contamination, 78 crop and landuse types were considered by grouping them into ten categories (CAFO, citrus, deciduous fruits and nuts, field crops, forage, native, pasture, truck crops, urban, and vineyards). Vadose zone thickness, soil type, well construction information, well proximity to dairies, and dominant landuse near the well were considered. In the Stan/Mer area, elevated nitrate levels in domestic wells most strongly correlate with the combination of very shallow (≤21 m) water table and the presence of either CAFO derived animal waste applications or deciduous fruit and nut crops (synthetic fertilizer applications). In Tulare County, statistical data indicate that elevated nitrate levels in domestic well water are most strongly associated with citrus orchards when located in areas with a very shallow (≤21 m) water table. Kings County had relatively few nitrate MCL exceedances in domestic wells, probably due to the deeper water table in Kings County.

Spatial Variability and Transport of Nitrate in a Deep Alluvial Vadose Zone

Little empirical evidence exists about the spatial distribution of NO 3 –N in deep vadose zones and about the associated fate and transport of NO 3 –N between the root zone and the water table. We investigated NO 3 –N occurrence in a deep alluvial vadose zone and its relation to geologic site characteristics, hydraulic properties, and fertilizer application rates via an intensive three-dimensional core-sampling campaign beneath an irrigated orchard in semiarid Fresno County, California. Statistical and geostatistical analyses were used to determine spatial variability of NO 3 –N and water content, to estimate total NO 3 –N mass in the vadose zone beneath each of three fertilizer treatments, and to compare NO 3 –N occurrence with that predicted from standard agronomic analysis of N and water flux mass balances. Vadose zone NO 3 was highly variable and lognormally distributed. Fertilizer treatment had a significant effect on NO 3 –N levels in the vadose zone. In all cases, deep vadose zone N mass estimated by kriging measured data totaled only one-sixth to one-third of the mass predicted by the N and water flux mass balance approach. Vadose zone denitrification estimates could not account for this discrepancy. Instead, the discrepancy was attributed to highly heterogeneous flux conditions that were not accounted for by the mass-balance approach. The results suggest that spatially variable vadose zone flow conditions must be accounted for to better estimate the potential for groundwater NO 3 loading.

Conditional stochastic analysis of solute transport in heterogeneous, variably saturated soils

A method is developed for the conditional (Monte Carlo) simulation of steady stateflow and transient transport from point sources in variably saturated porous media. It combines the geostatistical method, a linearized approximation of the soil water tension perturbation solution, and afinite element numerical model. The method is used to investigate the usefulness of conditional simulation for predicting solute transport under a variety of sampling network designs applied to a number of hypothetical soils. Saturated hydraulic conductivity data yield the largest reduction of conditional uncertainty in relatively wet soils with mild heterogeneities. In highly heterogeneous soils or under dry conditions, soil water tension data by themselves, taken at a sampling density of one to two correlation scales along the expected mean travel path, can greatly reduce prediction uncertainty about solute concentration. Parameter uncertainty about statistical properties of the independent random variables becomes less important as the number of conditioning data increases. However, even with a very high number of sampling data, uncertainty of predicted concentration levels remains significant.

Water flow and solute spreading in heterogeneous soils with spatially variable water content

Although the spatial variability of water content is expected to be significant in heterogeneous, unsaturated media, its effect on solute transport has been neglected in most stochastic analyses. In this work we develop, evaluate, and numerically verify an analytical model describing the statistical moments of water content, soil water flux, soil water velocity, and solute spreading in the unsaturated zone under conditions of variable soil water content. Results are presented for a wide range of soil conditions. It is shown that soil water content variability increases with soil water tension and decreases with soil textural tortuosity parameter m. As expected, the water content variability also increases with those of soil texture and saturated hydraulic conductivity. However, at or above field capacity (near saturation) the water content variability is generally negligible. The water content variability is very large under dry conditions, even if the soil is relatively homogeneous. In the same soil, longitudinal macrodispersivity is generally smaller when including than when neglecting the variability in water content. Variable water content enhances longitudinal macrodispersivity only in soils with large horizontal anisotropy or soils with m < 2. In this case the enhancement is more significant in drier soils and at the early stages of plume displacement (less than 10 λƒ). With travel distance the longitudinal macrodispersivity in the presence of variable water content converges to that in the absence of it.

Environmental occurrence and shallow ground water detection of the antibiotic monensin from dairy farms

Pharmaceuticals used in animal feeding operations have been detected in various environmental settings. There is a growing concern about the impact on terrestrial and aquatic organisms and the development of antibiotic-resistant strains of microorganisms. Pharmaceutical use in milking cows is relatively limited compared with other livestock operations, except for the ionophore monensin, which is given to lactating cows as a feed. By weight, monensin can be the most significant antibiotic used in a dairy farm. This study investigates the potential of monensin to move from dairy operations into the surrounding ground water. Using two dairy farms in California as study sites, we twice collected samples along the environmental pathway-from flush lanes, lagoon waters, and shallow ground water beneath the dairies and beneath its associated manured fields. Monensin concentrations were determined using solid-phase extraction and liquid chromatography-tandem mass spectrometry with positive electrospray ionization. Monensin was detected in all of the flush lane and lagoon water samples. Theoretical maximum concentration estimated from the actual dosing rate and the theoretical excretion rate assuming no attenuation was one order of magnitude greater than observed concentrations, suggesting significant attenuation in the manure collection and storage system. Monensin was also detected, at levels ranging from 0.04 to 0.39 microg L(-1), in some of the ground water samples underneath the production area of the dairy but not from the adjacent manured fields. Concentrations in ground water immediately downgradient of the lagoons were one to two orders of magnitude lower than the concentrations detected in lagoons, suggesting attenuation in the subsurface. The data suggest the possibility of monensin transport into shallow (2-5 m) alluvial ground water from dairy management units, including manure storage lagoons and freestalls occupied by heifers, lactating cows, and dry cows.