Groundwater Quality of the Dakota Aquifer
Water Quality Basics
Water quality is a major factor in the development of a water supply, and quality problems can arise from a multitude of sources, both natural and anthropogenic. As groundwater moves through sediments and rocks, it dissolves some of the more soluble minerals, adding to the water’s total dissolved solids (TDS). In general, deeper aquifers contain older groundwater that has been in contact with rocks longer, so they have higher concentrations of various dissolved solids. Carbonate minerals are the most commonly soluble and contribute dissolved calcium (Ca), magnesium (Mg), and bicarbonate (HCO3) to groundwater. Sulfur-bearing minerals like gypsum (CaSO4·2H2O) and pyrite (FeS2), while less common, add sulfate (SO4) to the water. Other minerals and buried organic matter can add dissolved or gaseous constituents such as iron (Fe), manganese (Mn), arsenic (As), hydrogen sulfide (H2S), ammonia (NH3), methane (CH4), and radioactive compounds like radium (Ra) and radon (Rn). Because water is an excellent solvent, it retains a signature of the geologic materials that it passes through over time. These natural constituents can affect the taste, smell, and color of water, its usefulness for various purposes, and human and animal health.
Table 1 summarizes some commonly occurring chemical constituents and properties that can cause problems in Iowa drinking water. Contaminants that affect health must meet legally enforceable primary Maximum Contaminant Level (MCL) standards for public water supplies, while contaminants that affect the aesthetic quality of water are unregulated and use Secondary Maximum Contaminant Levels (SMCLs) to define acceptable levels of contamination based on taste, odor, color and certain other non-aesthetic effects of drinking water. Contaminants that affect health include bacteria, nitrate (NO3), pesticides, radionuclide, organic chemicals, arsenic, and lead (Pb).
Table 1. Commonly occurring constituents and their significance in drinking water (modified from Iowa’s Groundwater Basics by Prior, et al., 2003).
Contaminants that affect the aesthetic quality of water, but do not affect health, at least in small quantities, include sulfate, total dissolved solids, calcium, magnesium, hydrogen sulfide, iron, manganese, and iron bacteria. SMCLs for these constituents are often exceeded for long periods without obvious detrimental effects, although the water may have a bad odor and be unpalatable. In many areas, the best water that is locally available may have aesthetic problems, but still be widely used. For constituents that affect health, the regulated MCLs can not be exceeded legally, even for short periods.
Publications about water quality and its effects on health are available from the U.S. Environmental Protection Agency (USEPA) at www.epa.gov/safewater/, the U.S. Geological Survey (USGS) at water.usgs.gov/owq/, and the Iowa Geological and Water Survey by clicking here.
The Dakota Aquifer
The first aquifer to be studied for the new Water Resources Management program is the Dakota, which is used for rural and public water supplies in western Iowa (Figure 1). This aquifer is the youngest and shallowest of the bedrock aquifers in Iowa, and is composed of two members: the upper Woodbury Member shales and very fine- to fine-grained sandstones, and the underlying Nishnabotna Member fine- to very course-grained sandstones. These deposits formed in riverine environments 100 million years ago. Woodbury rocks form a minor aquifer with low to moderate yields, while Nishnabotna rocks form a major aquifer capable of yielding greater than 1,500 gallons per minute (gpm) in some areas. Because of the greater continuous areal extent and higher yields, the initial modeling and groundwater resource evaluations concentrated on the lower part of the Dakota Aquifer within the 16 counties in northwest Iowa.
Figure 1. Area of occurrence and significant use of the Dakota Aquifer in western Iowa (modified from Iowa’s Groundwater Basics by Prior, et al., 2003).
The thickness of the aquifer sandstones varies widely, but generally ranges from 200 to 300 feet throughout much of the study area. The sandstones are confined over most of the study area by 200 to 400 feet of clay-rich glacial till as well as by thick shale, siltstone, thin chalky limestone, and lignite (low-grade coal). Most wells developed in the aquifer range from 100 to 600 feet deep in the area. The confining beds underlying the aquifer include Dakota shales, undifferentiated Paleozoic rocks, and Precambrian crystalline rock.
For the Dakota Aquifer groundwater quality evaluation, the upper and lower members of the aquifer are not distinguished because there is currently not enough well data to determine if there are consistent differences in water quality between the upper and lower members. It appears that there may be differences, and that a major factor influencing the water quality may be the depth of the aquifer and the type of materials overlying it. In general, the lower part of the Dakota has greater yield potential, but probably poorer natural water quality. For practical purposes, domestic supplies often use the upper portion of the aquifer because drilling costs are lower, and they do not need large yields. Public and industrial users that need greater yields must use the lower portion of the aquifer, even if the water quality is poorer.
The new Iowa Geological and Water Survey Groundwater Quality Database
The Dakota Aquifer groundwater quality evaluation is the first water resource study to use water quality information from a new database that is available from the Iowa Geological and Water Survey’s Natural Resources Geographic Information System (NRGIS) Library (www.igsb.uiowa.edu/nrgislibx/) as a downloadable shapefile named GW_Quality.zip. This GIS groundwater quality database, which can be thought of as a map layer, or coverage, was constructed to characterize Iowa’s aquifers and determine if contamination from human activity has increased in any of the aquifers in recent years. The database is divided into two separate map layers named “General” and “Contaminant,” with the General map layer containing naturally occurring constituents including metals, physical characteristics (total dissolved solids, pH, etc.), and radionuclides, and the Contaminant map layer accommodating nutrients, man-made contaminants such as volatile organic compounds, and pesticides. The map layers include geographically indexed, or geo-referenced, data from wells completed in all of Iowa’s major aquifers. Both layers contain over 8,000 analyses of unfinished water, collected from over 2,000 wells, with a combined total of over 300 sampled parameters. The samples were collected over many years by numerous individuals from private, public, and government sectors from public and private wells across Iowa. Most samples are from public water supplies and were analyzed by the University Hygienic Laboratory (UHL), but samples from a variety of projects, including aquifer and water studies, contaminant plume mapping, and maximum contaminant level compliance monitoring, and analyses from other laboratories are also included in the map layers.
The General and Contaminant data sets each currently contain 728 different water quality analyses collected from 139 wells completed in the Dakota Aquifer within the study area in northeast Iowa.
Dakota Aquifer Groundwater Quality
The Dakota Aquifer groundwater quality evaluation focuses on naturally occurring contaminants, since anthropogenic, or man-made, contaminants are usually not found in water from the aquifer because it is protected from surface contamination in most areas by thick overlying glacial drift and shales. Some Dakota wells penetrate the full extent of the aquifer and some are completed in only the upper part. As mentioned, private wells tend to be completed in the upper part, while public and industrial wells usually penetrate the full extent of the aquifer to obtain greater yields. Differences in water quality between the upper and lower parts of the aquifer may be significant in areas where the upper part of the aquifer is not protected from relatively rapid recharge of water by low-permeable materials like thick glacial tills and shales, while the lower part of the aquifer is protected by low-permeable shales and lignites.
As discussed, there are many elements dissolved in water, including organic and inorganic materials, gases and metals. Concentrations of these dissolved substances are often reported as mg/L, which is equivalent to parts per million (ppm). Concentrations of trace amounts of dissolved constituents are often listed as micrograms per liter (μg/L), or parts per billion (ppb).
Although TDS are generally not considered a health hazard, water treatment is recommended when TDS concentrations exceed the EPA’s 500 mg/L SMCL, and further testing may be warranted, as water with a high TDS concentration may indicate elevated levels of ions that may pose a health concern, such as aluminum, arsenic, copper, lead, nitrate, and others.
Total dissolved solids are often used as an indicator of the aesthetic characteristics of drinking water, and as an aggregate indicator of the presence of a broad array of chemical contaminants. For drinking purposes, water is considered good when it contains less than 500 mg/L of TDS, fair when it contains 500 to 1,000 mg/L, and poor when greater than 1,000 mg/L of TDS are present.
The water quality of the Dakota Aquifer tends to be fair to poor throughout most of the study area, and good to fair towards the corners of the study area (Figure 2). The areas of poorer water quality result from high concentrations of TDS (between 500 and 3,000 mg/L), particularly sulfate and calcium carbonate, which are common minerals picked up by groundwater in contact with the confining layers above. Areas of better water quality are found where confining layers are thin and porous pathways allow for more rapid recharge by less mineralized, younger water. This occurs most notably in the southwest, southeast, and northeast portions of the study area where the aquifer is closer to the land surface. As discussed, older groundwater contains more TDS because the water has been in contact with the surrounding materials longer, allowing more time for dissolution of minerals into the water.
Figure 2. Water quality, from good to poor, based on total dissolved solids concentration, in mg/L, in water from the Dakota Aquifer in northwest Iowa.
The chemical characteristics of groundwater are determined by factors such as the mineral content of the soil and aquifer materials through which the water passes; the rate of groundwater movement; the length of time that water remains in an aquifer; the chemistry of the water when it was trapped during sediment deposition; the chemical and physical changes that take place in aquifer materials after they are deposited; and the mixing of groundwater in an aquifer with water from adjacent hydrogeologic units and/or water from the land surface.
If yields from the Dakota Aquifer are significantly increased in the future, the quality of the water in the aquifer may be degraded by upward leakage from underlying Paleozoic aquifers. Large withdrawals may reverse the natural flow from the Dakota into the underlying aquifers. This may currently be occurring near the city of LeMars, where concentrations of TDS and sulfate are abnormally high, and the potentiometric surface of the Dakota is below that of the underlying Paleozoic aquifers which contain water with higher concentrations of TDS and sulfate.
Although the groundwater quality of the Dakota Aquifer is relatively poor throughout much of the study area, many communities, industries, farms, and homes use it because there are no other alternatives. Domestic supplies often use the upper portion of the aquifer because drilling costs are lower, and they do not need large yields, while public and industrial users that need greater yields use the lower portion of the aquifer, even if the water quality is poorer. With treatment, contaminants that affect health can usually be reduced to meet MCLs, and contaminants that affect the aesthetic qualities of water can often be reduced to acceptable levels.
As more wells are completed in the aquifer and more stratigraphic, construction, and water-quality data are interpreted and entered into our databases, our knowledge of the Dakota Aquifer will improve and our evaluation of it will be refined.
To view the complete report, click on the link below.