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 ≥ 10 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 >15 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 < 0.02) to ∼18.3 and 18.8 mg N/L, respectively, during >20 years of management. Average annual concentrations in 10 MAs are increasing (p < 0.05) 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.

Atrazine and nitrate in public drinking water supplies and non-hodgkin lymphoma in nebraska, USA.

A secondary analysis of 1999-2002 Nebraska case-control data was conducted to assess the risk of non-Hodgkin lymphoma (NHL) associated with exposure to nitrate- and atrazine-contaminated drinking water. Water chemistry data were collected and weighted by well contribution and proximity of residence to water supply, followed by logistic regression to determine odds ratios (OR) and 95% confidence intervals (CI). We found no association between NHL risk and exposure to drinking water containing atrazine or nitrate alone. Risk associated with the interaction of nitrate and atrazine in drinking water was elevated (OR, 2.5; CI, 1.0-6.2). Risk of indolent B-cell lymphoma was higher than risk of aggressive B-cell lymphoma (indolent: OR, 3.5; CI, 1.0-11.6 vs. aggressive: OR, 1.9; CI, 0.6-5.58). This increased risk may be due to in vivo formation and subsequent metabolism of N-nitrosoatrazine. A larger study is warranted to confirm our findings.

Risk-managed approach for routing petroleum pipelines: Keystone XL pipeline, Nebraska.

TransCanada's proposed international crude oil pipeline route over sensitive, relatively pristine, subirrigated land underlain by the Ogallala aquifer led to increased scrutiny and eventual rejection of the Keystone XL pipeline. Pipeline routing could be made much more acceptable by adopting risk-managed routes that lessen the potential to adversely impact high-quality groundwater and, should a release occur, decrease the longevity of hazardous groundwater contaminants. Threats to water quality are taken quite seriously in states like Nebraska where 85% of the population depend on groundwater for potable water.

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 approximately 23,800 producer reports. The terrace, comprising approximately 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 approximately 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 < 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 approximately 15% of the approximately 28,300 furrow-irrigated terrace hectares (approximately 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.

Transport and degradation of ethanol in groundwater.

Ethanol is rapidly replacing methyl tert-butyl ether (MtBE), the primary fuel oxygenate in the US, and ethanol releases from spills and leaky underground storage tanks (LUSTs) are anticipated. Ethanol has received little attention as a potential groundwater contaminant. This study investigates the fate and transport of ethanol under transient conditions in a sand and gravel aquifer. A pulse containing approximately 220 mg L-1 ethanol and 16 mg L-1 bromide was injected into the shallow sand and gravel aquifer and monitored to estimate its persistence and transport. The plume was monitored for 2.5 months using downgradient multilevel samplers (MLSs). Values for ethanol retardation were measured from ethanol and bromide breakthrough data and compared to estimates using published Koc values for low carbon aquifer sediments (foc=10 microg C g-1 sediment). Ethanol transport was not retarded (R=0.99). A 3-dimensional model reasonably simulated bromide and ethanol breakthrough curves. An average first-order decay constant was estimated to be 0.32 d-1 (t1/2=2.2 d). At the second fence, 75% of the injected bromide and less than 3% of ethanol remained in the plume. Monitored terminal electron acceptor concentrations demonstrated that the majority of the ethanol was transformed by anaerobic processes other than denitrification and sulfate reduction.

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.

Enhanced in situ denitrification for a municipal well.

In 37% of small community systems in Nebraska at least one sample exceeded the drinking water standard for nitrate of 10 mg N L(-1) during the period from 1982 to 1998 (US Bureau of Reclamation, US Department of the Interior, 1999). In this experiment a daisy well system was designed to promote denitrification in the radial capture zone of a municipal well with nitrate-N levels> 10mg L(-1), and thereby bring the nitrate concentration into compliance. The remediation design consisted of eight 15 cm diameter outer perimeter reduction wells and eight 5 cm diameter inner perimeter oxidation wells which are located roughly 18 and 9 m, respectively, from the municipal well, which serves as the extraction well. Endemic microbes are stimulated by pulsing separate injections of acetate-C and nitrate contaminated water (C:N = 1.2) to enhance denitrification in the capture zone. Water was extracted from the municipal well at 6.6 L s(-1) (liters per second). A 45% nitrate reduction occurred in municipal well samples when the total acetate-C input was increased by lengthening the acetate pulse from 1.0 to 1.5 h (C:N=1.8). Nitrate concentration stabilized at about 6.3 mg NO3-N L(-1) for two weeks during alternating acetate pulse lengths. The in situ denitrification process was sustained for three months without evidence of clogging. Results from this experiment indicated that the extracted water was in compliance with respect to nitrate, nitrite, trihalomethanes, turbidity, and total and fecal coliforms; however, the total plate count exceeded the maximum permissible limit (500 cfu/mL).

Herbicides in ground water beneath Nebraska's Management Systems Evaluation Area.

Profiles of ground water pesticide concentrations beneath the Nebraska Management Systems Evaluation Area (MSEA) describe the effect of 20 yr of pesticide usage on ground water in the central Platte Valley of Nebraska. During the 6-yr (1991-1996) study, 14 pesticides and their transformation products were detected in 7848 ground water samples from the unconfined water table aquifer. Triazine and acetamide herbicides applied on the site and their transformation products had the highest frequencies of detection. Atrazine [6-chloro-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine-2,4,-diamine] concentrations decreased with depth and ground water age determined with 3H/3He dating techniques. Assuming equivalent atrazine input during the past 20 yr, the measured average changes in concentration with depth (age) suggest an estimated half-life of >10 yr. Hydrolysis of atrazine and deethylatrazine (DEA; 2-chloro-4-amino-6-isopropylamino-s-triazine) to hydroxyatrazine [6-hydroxy-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine-2,4-diamine] appeared to be the major degradation route. Aqueous hydroxyatrazine concentrations are governed by sorption on the saturated sediments. Atrazine was detected in the confined Ogallala aquifer in ultra-trace concentrations (0.003 microg L(-1)); however, the possibility of introduction during reverse circulation drilling of these deep wells cannot be eliminated. In fall 1997 sampling, metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl) acetamide] was detected in 57% of the 230 samples. Metolachlor oxanilic acid [(2-ethyl-6-methylphenyl)(2-methoxy-1-methylethyl) amino]oxo-acetic acid] was detected in most samples. In ground water profiles, concentrations of metolachlor ethane sulfonic acid [2-[(ethyl-6-methylphenyl)(2-methoxy-1-methylethyl)amino]-2-oxo-ethanesulfonic acid] exceeded those of deethylatrazine. Alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide] was detected in <1% of the samples; however, alachlor ethane sulfonic acid [2-[(2,6-diethylphenyl)(methoxymethyl)amino]-2-oxoethanesulfonic acid] was present in most samples (63%) and was an indicator of past alachlor use.

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 Systems Evaluation 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) acetamidel) annually; the alfalfa field was untreated. Ground water samples were collected three times annually from 16 depths of 31 multilevel 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 <0.5 microg L(-1). Increases in deethylatrazine (DEA; 2-chloro-4-amino-6-isopropylamino-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.