The High Plains aquifer, part two

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Part one of this article discussed the importance of the High Plains aquifer, one of the largest aquifers in the world, the use of which has transformed the High Plains into a major agricultural region that sustains more than a quarter of U.S. agricultural production. The aquifer is largely not a renewable resource, and water levels in the aquifer have declined substantially since the 1950s.

Part two, below, discusses the results of the U.S. Geological Survey study of groundwater quality in the High Plains.

By Jason Gurdak, Ph.D.

Major findings and implications from the USGS study of groundwater quality

In general, groundwater in the High Plains aquifer currently meets federal and state guidelines for drinking-water quality. However, many factors, such as water use and chemical use, can affect water quality over time. An understanding is therefore needed on the timing and magnitude of the transport of chemicals from the land surface through the unsaturated zone and to groundwater for informed future management and development of this limited resource. Seven of the most important major findings and related implications from the USGS study of groundwater quality in the High Plains are present below.

Availability and sustainability of the High Plains aquifer are a function of groundwater quantity and quality

Groundwater is the primary source of water used in the High Plains; thus, knowledge about groundwater availability and sustainability are essential for the informed management of this limited resource. Groundwater availability and sustainability are functions of many factors, one of which is water quality. Understanding groundwater quality is important because it directly affects how water can be used. Water quality generally has been overlooked in the High Plains because the primary focus has been on obtaining a sufficient water supply, and it has been broadly assumed that the aquifer contains high-quality water. For the most part, results from the USGS study support that assumption.

Implications:

  • Groundwater quality, particularly regarding elevated nitrate or dissolved-solids concentrations, may be a limiting factor for some intended uses, such as drinking- or irrigation-water supply at local and, in some instances, subregional scales.
  • Groundwater quality, particularly regarding elevated nitrate concentrations in recently recharged groundwater, is changing over time and, because of the slow rates of water movement in the aquifer, could affect groundwater availability for decades and even millennia.

Conversion of rangeland to irrigated cropland affects water quality

Nitrogen and pesticide applications on irrigated cropland result in substantially more nitrate and pesticides being transported to the water table at irrigated cropland settings than at rangeland settings. In some locations, however, the change in recharge chemistry following conversion of rangeland to cropland results from the mobilization by irrigation water of large natural nitrate and chloride deposits in the unsaturated zone (above the water table).

Implications:

  • Reducing the acreage of rangeland that is converted to irrigated cropland in areas that contain large natural subsurface nitrate and chloride deposits (currently not delineated across the entire High Plains aquifer) is likely to be an effective way to reduce the dissolution and transport of natural chemical constituents in recharge water and resulting adverse effects on water quality in the aquifer.
  • Implementing efficient agricultural-chemical application and irrigation technologies in areas not currently implemented to reduce deep percolation of irrigation water and chemicals under cropland is likely to be an effective way to reduce the adverse effects on the quality of the recently recharged groundwater.

Chemical transport to the water table follows fast and slow paths

Chemical transport from land surface to the water table occurs by fast and slow paths through the unsaturated zone. In most locations studied, estimated chemical transit times from land surface to the water table exceeded the period of agricultural activity, which implies that agricultural chemicals should not yet be present at the water table. In fact, agricultural chemicals are commonly detected at the water table beneath irrigated cropland.

This apparent discrepancy is explained by localized fast or preferential flow paths that enable water and chemicals to move quickly through the unsaturated zone to the water table. Fast paths are most likely to be present beneath topographic depressions in the land surface in which surface runoff from irrigation or precipitation collects. Slow paths are most likely to be present in areas of fine-grained sediments or beneath flat terrain where surface water does not collect. Along fast paths, water and chemicals from land surface may reach the water table in months to decades. Along slow paths, water and chemicals from land surface are likely to reach the water table in centuries to millennia.

Implications:

  • Reducing the amount of untreated agricultural and urban runoff to topographic depressions or other fast-path zones is likely to reduce the rapid transport of contaminants to the water table and be an effective management strategy toward minimizing groundwater contamination.

Important differences exist between the quality of shallow groundwater and deep groundwater

Changes in water quality have occurred over time that may affect the sustainability of the groundwater resource in the High Plains aquifer. Important spatial differences in the concentrations of dissolved solids, nitrate, pesticides, arsenic and other constituents were observed between the quality of shallow and relatively young groundwater and the quality of deep and relatively old groundwater.

Dissolved-solids concentrations in shallow groundwater generally increased from north to south in the High Plains aquifer and were significantly greater in New Mexico and Texas (median of 800 milligrams per liter [mg/L]) than in the Colorado, Kansas and Nebraska (medians of about 450 to 500 mg/L).

Nitrate was above local background concentrations (4 mg/L) as nitrogen in shallow groundwater was detected throughout the High Plains aquifer. Nitrate concentrations in shallow groundwater were greater than the background concentration in 90 percent of samples from the northern, 60 percent of samples from the central and 55 percent of samples from the southern High Plains.

Although widely detected, pesticide concentrations in shallow groundwater were less than U.S. Environmental Protection Agency (EPA) drinking-water standards in all but two of 119 samples. Atrazine and its degradate deethylatrazine were the most commonly detected pesticide compounds.

Concentrations of arsenic in shallow groundwater were significantly greater in samples from the southern High Plains than in samples from the northern High Plains. About 40 percent of the samples in the southern High Plains exceeded the EPA arsenic drinking-water standard of 10 mg/L, whereas none exceeded the standard in the northern High Plains.

The quality of deeper groundwater that is used by many private, public-supply and irrigation wells in the High Plains generally is of suitable quality for most uses, although some spatial differences in water quality are observed across the region. Based on EPA drinking-water standards, the quality of deeper groundwater decreases from the northern High Plains to the southern High Plains, where the water contains greater concentrations of dissolved solids, chloride, nitrate, fluoride, manganese, arsenic and uranium. These elevated constituent concentrations in deeper groundwater from the southern High Plains are the result of natural processes, such as water/rock interactions, and human activities, such as the mixing of high-quality groundwater with lower-quality water from underlying hydrogeologic formations, induced by pumping of high-capacity wells.

Implications:

  • The quality of groundwater near the water table may not be currently suitable for human consumption in some locations of the aquifer because of elevated concentrations of salts, nitrate and (or) pesticides. However, the quality of groundwater in deeper parts of the aquifer, with some exceptions, currently is generally suitable for human consumption.
  • The quality of groundwater that is influenced by mixing with brackish (water containing elevated salt concentrations) surface water or water from underlying hydrogeologic formations may not be suitable for irrigation or drinking-water supply in some locations of the aquifer because of the elevated concentrations of dissolved solids.
  • Groundwater remediation is expensive, slow and impractical across regional-scale aquifers. Therefore, management practices that prevent groundwater contamination are likely to be a more effective way to maintain the availability and sustainability of groundwater in the High Plains aquifer for the intended uses for human consumption and agricultural practices.
  • Collecting long-term monitoring data is likely to be an effective way to detect gradual temporal trends and to provide early warning of water-quality problems (nitrate, for example) for which the aquifer may have limited natural-attenuation capacity.

Mixing of groundwater by high-capacity wells with long or multiple screens adversely affects water quality

The quality of deeper groundwater from private, public-supply and irrigation wells, which typically are completed using one long screen or multiple screens across much of the saturated thickness of the High Plains aquifer, is sometimes affected by mixing with poorer-quality water from the water table. The mixing is caused by leakage through long or multiple well screens and long-term pumping of high-capacity public-supply and irrigation wells. In other instances, pumping of high-capacity wells causes water in the aquifer to be affected by mixing with saline water from underlying bedrock aquifers or with water from streams that is of poorer quality, based on EPA drinking-water standards.

Implications:

The following management strategies are likely to help reduce adverse effects on the quality of water in deep zones of the aquifer caused by mixing processes:

  • Reduce the screen length of public-supply and irrigation wells, and optimize the spatial and temporal distribution of pumping to minimize mixing and water-table drawdown;
  • Eliminate the practice of screening wells across confining layers in the aquifer; and
  • Practice screening wells below confining layers in the aquifer where contaminant sources at the land surface are a concern and practice screening wells above confining layers in the aquifer where contaminant sources in underlying bedrock are a concern.
  • Proper construction of wells to limit the movement of low-quality groundwater above and (or) below a well screen will result in maintaining a higher-quality groundwater supply for a variety of uses.

The aquifer has limited ability to naturally remove some contaminants

The High Plains aquifer is limited in its ability to naturally remove some contaminants, such as nitrate. Denitrification, the primary natural chemical process for removing nitrate in groundwater, generally occurs very slowly in the aquifer and would require hundreds to thousands of years to lower nitrate concentrations by just 1 mg/L as nitrogen (N) in many locations (note: 10 mg/L of nitrate (as N) is the EPA drinking-water standard). Additionally, because water residence times in the aquifer are long (several thousand years in some locations), simply flushing nitrate from the aquifer could take many years. These results highlight the importance of managing land use in the High Plains to minimize the amount of nitrate entering the aquifer.

Implications:

  • In the indefinite future, the aquifer will continue to be subject to the effects of mobilized agricultural chemicals on groundwater quality from ongoing irrigation, conversion of rangeland to cropland or climate variability.
  • The long transit times of nitrate through the unsaturated zone will undoubtedly delay future improvements in water quality from implementation of best management land-use practices.

The quality of most water produced by private, public-supply and irrigation wells is suitable for the intended uses

The good news is that the quality of groundwater from deeper in the High Plains aquifer, where most private, public-supply and irrigation wells are screened, is generally suitable for drinking and as irrigation water. Comparison of private well water quality to EPA national primary and secondary drinking-water standards indicates that water from the Ogallala Formation in the northern and central High Plains had the best water quality, whereas water from the Ogallala Formation in the southern High Plains had the poorest quality. Most exceedances of primary and secondary drinking-water standards were those for dissolved solids, nitrate, arsenic, fluoride, iron, manganese and nitrate. The most frequently detected pesticide compounds were atrazine and deethylatrazine, and the most frequently detected volatile organic compound was chloroform. None of the pesticide compounds or volatile organic compounds exceeded a primary drinking-water standard.

Implications:

  • If contaminated, the deep zones in the aquifer, in which production wells are screened, are not likely to be remediated quickly because of slow recharge rates, long water residence times in the aquifer and slow rates of contaminant degradation.

The complete USGS report is available at http://pubs.usgs.gov/circ/1337/. Additional information about the USGS study on groundwater quality of the High Plains aquifer is available at http://co.water.usgs.gov/nawqa/hpgw/HPGW_home.html.

 

This article includes modified excerpts from the U.S. Geological Survey report titled “Water Quality in the High Plains Aquifer,” USGS Circular 1337, available online at http://pubs.usgs.gov/circ/1337/.

 

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