Herbicide Loading to Shallow Ground Water beneath Nebraska’s Management Systems Evaluation Area

Better management practices can counter deterioration of ground water quality. From 1991 through 1996 the influence of improved irrigation practices on ground water pesticide contamination was assessed at the Nebraska Management SystemsEvaluation Area. Three 13.4-ha corn (Zea mays L.) fields were studied: a conventional furrow-irrigated field, a surge-irrigated field and a center pivot–irrigated field, and a center pivot–irrigated alfalfa (Medicago sativa L.) field. The corn fields received one identical banded application of Bicep (atrazine [6-chloro-N-ethyl-N’-(1-methylethyl)-1,3,5-triazine-2,4,-diamine] _ metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl) acetamide]) annually; the alfalfa field was untreated. Ground water samples were collected three times annually from 16 depths of 31 multi-level samplers. Six years of sample data indicated that a greater than 50% reduction in irrigation water on the corn management fields lowered average atrazine concentrations in the upper 1.5 m of the aquifer downgradient of the corn fields from approximately 5.5 to –1. Increases in deethylatrazine (DEA; 2-chloro-4-amino-6-isopropyl- amino-s-triazine) to atrazine molar ratios indicated that reducing water applications enhanced microbial degradation of atrazine in soil zones. The occurrence of peak herbicide loading in ground water was unpredictable but usually was associated with heavy precipitation within days of herbicide application. Focused recharge of storm runoff that ponded in the surge-irrigated field drainage ditch, in the upgradient road ditch, and at the downgradient end of the conventionally irrigated field was a major mechanism for vertical transport. Sprinkler irrigation technology limited areas for focused recharge and promoted significantly more soil microbial degradation of atrazine than furrow irrigation techniques and, thereby, improved ground water quality

Groundwater Quality and Policy Options in Nebraska

Potential contaminants and the occurrence of groundwater contamination in Nebraska are discussed. An overview of Nebraska\u27s policy response to groundwater quality reveals that the policy has been fragmentary and generally reactive. Although a comprehensive groundwater quality protection strategy is needed if the groundwater is to be protected from potential point and nonpoint sources of contamination, it must recognize the site-specific nature of most groundwater contamination. The Nebraska Chemigation Act and the Petroleum Products and Hazardous Substances Storage and Handling Act passed in 1986 were the first comprehensive legislation addressing prevention of point source contamination. Proactive policies for the prevention of nonpoint groundwater contamination are an economic necessity in today\u27s political climate

Long-Term Response of Groundwater Nitrate Concentrations to Management Regulations in Nebraska’s Central Platte Valley

The impact of 16 years (1988–2003) of management practices on high groundwater nitrate concentrations in Nebraska’s central Platte River valley was assessed in a 58,812-ha (145,215-ac) groundwater quality management area intensively cropped to irrigated corn (Zea mays L.). Crop production and groundwater nitrate data were obtained from ~23,800 producer reports. The terrace, comprising ~56% of the study area, is much more intensively cropped to irrigated corn than the bottomland. From 1987 to 2003, average groundwater nitrate concentrations in the primary aquifer beneath the bottomland remained static at ~8 mg N/l. During the same period, average groundwater nitrate concentrations in the primary aquifer beneath the terrace decreased from 26.4 to 22.0 mg N/l at a slow, but significant (p \u3c 0.0001), rate of 0.26 mg N/l/year. Approximately 20% of the decrease in nitrate concentrations can be attributed to increases in the amount of N removed from fields as a consequence of small annual increases in yield. During the study, producers converted ~15% of the ~28,300 furrow-irrigated terrace hectares (~69,800 ac) to sprinkler irrigation. The conversion is associated with about an additional 50% of the decline in the nitrate concentration, and demonstrates the importance of both improved water and N management. Average N fertilizer application rates on the terrace were essentially unchanged during the study. The data indicate that groundwater nitrate concentrations have responded to improved management practices instituted by the Central Platte Natural Resources District

DEVELOPMENT OF A QUALITY-ASSESSED AGRICHEMICAL DATABASE FOR MONITORING ANTHROPOGENIC IMPACTS ON GROUND-WATER QUALITY

The Quality-Assessed Agrichemical Contaminant Database for Nebraska Ground Water is a unique repository of nitrate and pesticide data collected by federal, state, and local agencies. Each contaminant concentration in the database has been evaluated based upon well-defined criteria that address completeness of the well-attribute data, analytical method and field and laboratory quality control practices and assigned to one of five quality levels. The quality assessment level always accompanies the contaminant concentration so that the end-user knows the quality assurance effort expended in the acquisition of the data, can select comparable data, and choose data whose quality assurance effort is commensurate with project objectives. The database can be viewed and queried on-line; downloaded in its entirety; or imported to a spreadsheet or a geographic information system. Setting criteria for data quality and assessing the level of quality have resulted in significant increases in the percentages of high quality (Levels 3–5) nitrate and pesticide data. These high quality data presently constitute 52% of the nitrate and 55% of the pesticide data

Variability of Anaerobic Animal Waste Lagoon delta\u3csup\u3e15\u3c/sup\u3eN Source Signatures

High ammonium-N concentrations derived from animal wastes stored and partially treated in earthen anaerobic lagoons at confined feeding facilities can seep to groundwater. δ15N-NH4+ values from +2.0 to +59.1‰ in 13 lagoons complicate identification of lagoon seepage as well as land-applied lagoon effluent in ground and surface waters. The spectrum of δ15N values requires site-specific isotope characterization of the potential source. Feed and fresh manure and urine δ15N values indicate that most N isotopic fractionation occurs after excretion. Lagoon management clearly affects enrichment. δ15N-total Kjeldahl N (TKN) and δ15N-NH4+ within each lagoon were not statistically different. δ15N-NH4+ within the top 1.5 m of the lagoons was spatially uniform (CV [coefficient of variation] \u3c5%)

Nebraska’s groundwater legacy: Nitrate contamination beneath irrigated cropland

A 31 year record of ~44,000 nitrate analyses in ~11,500 irrigation wells was utilized to depict the decadal expansion of groundwater nitrate contamination (N\u3e10 mg/L) in the irrigated corn-growing areas of eastern and central Nebraska and analyze long-term nitrate concentration trends in 17 management areas (MAs) subject to N fertilizer and budgeting requirements. The 1.3 M contaminated hectares were characterized by irrigation method, soil drainage, and vadose zone thickness and lithology. The areal extent and growth of contaminated groundwater in two predominately sprinkler-irrigated areas was only ~20% smaller beneath well-drained silt loams with thick clayey-silt unsaturated layers and unsaturated thicknesses \u3e15 m (400,000 ha and 15,000 ha/yr) than beneath well and excessively well-drained soils with very sandy vadose zones (511,000 ha and 18,600 ha/yr). Much slower expansion (3700 ha/yr) occurred in the 220,000 contaminated hectares in the central Platte valley characterized by predominately gravity irrigation on thick, well-drained silt loams above a thin (~5.3 m), sandy unsaturated zone. The only reversals in long-term concentration trends occurred in two MAs (120,500 ha) within this contaminated area. Concentrations declined 0.14 and 0.20 mg N/L/yr (p\u3c0.02) to ~18.3 and 18.8 mg N/L, respectively, during \u3e20 years of management. Average annual concentrations in 10 MAs are increasing (p\u3c0.05) and indicate that average nitrate concentrations in leachates below the root zone and groundwater concentrations have not yet reached steady state. While management practices likely have slowed increases in groundwater nitrate concentrations, irrigation and nutrient applications must be more effectively controlled to retain nitrate in the root zone

Sprinkler Irrigation: A Volatile Organic Compound Remediation Alternative

Sprinkler irrigation has the potential not only to cheaply and effectively remove volatile organic compounds (VOCs) from contaminated ground water but also use the water beneficially and eliminate the costly disposal of both the remediated water and the contaminants. Removal of VOCs from water involves volatilization, which releases the VOCs from the liquid phase to the gaseous phase. A conventionally designed sprinkler irrigation system was tested near Hastings, Nebraska, to assess its efficacy for removing VOCs in pumped ground water. VOCs in the ground water include 7,1,2-trichloroethylene( TCE), ethylene dibromide (EDB), 1,1,1-trichloroethane (TCA), and carbon tetrachloride (CT). Factors possibly influencing volatilization include nozzle size, system pressure, impact pad design, flow rate, and temperature

SPRINKLER IRRIGATON: A VOC REMEDIATION ALTERNATIVE

A sprinkler irrigation system was tested to assess its efficacy for volatilizing organic chemicals in pumped ground water. In field tests involving the analysis of more than 250 samples collected from beneath a spray irrigation system, removal rates of ethylene dibromide (EDB), 1,1,2-trichloroethylene (TCE), 1,1,1-trichloroethane (TCA), and carbon tetrachloride (CT) in samplers placed 0.5 m above the ground exceeded 95% in the vast majority of cases, and approached 100% for the more volatile chemicals. As predicted by Henry’s Law, CT, TCA, and TCE were significantly more volatile than EDB. Removal efficiencies of conventionally designed sprinkler irrigation systems were enhanced by using small aperture nozzles with impact pads designed to produce thin films of water. Droplet sizes produced by the various nozzle apertures and impact pad designs were measured using a phase Doppler particle analyzer and found to be one factor controlling volatilization. As predicted by the Clausius-Clapeyron equation, higher air temperatures appear to be associated with slightly increased volatilization. Using specialized stratified water droplet collectors, it was determined that longer droplet trajectories increased volatilization