Ground water is the principal source of water for public- and self-supplied domestic uses in Nebraska. Ground water supplied about 235 million gallons per day (Mgal/d) or about 78 percent of the estimated public-supplied water in Nebraska in 1990. The Platte River alluvial aquifer that underlies the Platte Valley is connected hydraulically to the Platte River. The aquifer is the single most important source of water for public supply. It provides about 117 Mgal/d, or about half of the total daily ground-water public-supply production for Nebraska. This aquifer supplies water to Nebraska's largest cities, including Omaha, Lincoln, Grand Island, and Kearney. In addition, ground water supplies about 83 percent, or about 1,017 Mgal/d, of the irrigation water in the valley [14].
Increased ground-water withdrawals from the alluvial aquifer may increase infiltration from the Platte River [15]. This increase in infiltration has the potential to change the ground-water quality near the river where many of the public-supply wells are located. The quality of the Platte River water can differ significantly with respect to location in the valley.
Stream-water quality in the Platte River with distance downstream.
Median concentrations of dissolved solids tend to decrease with distance downstream, whereas nitrate and pesticide concentrations increase with distance downstream. Median dissolved-solids concentrations for samples collected between 1992 and 1995 were 663 mg/L from the Tri-County Canal near North Platte, which represents stream water quality entering the Study Unit from the west, 578 mg/L from the Platte River near Grand Island near the middle of the Study Unit, and 380 mg/L from the Platte River at Louisville at the most downstream point where streamflow leaves the Study Unit. Median nitrate concentrations were 0.68 mg/L in samples from the Tri-County Canal, 1.4 mg/L from the Platte River near Grand Island, and 1.2 mg/L from the Platte River at Louisville. The median concentrations of 12 herbicides in surface water at these locations are listed in the table. Five of the twelve--acetochlor, alachlor, atrazine, cyanazine, and metolachlor-- indicate that herbicide loads increase with distance downstream in the Study Unit. Atrazine displayed the highest increase in median concentrations with 0.14 µg/L from Tri-County Canal near North Platte, 0.62 µg/L from the Platte River near Grand Island, and 0.91 µg/L from the Platte River at Louisville.
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Dissolved solids and pesticide concentrations in ground water are not greatly different than those in surface water, but nitrate concentrations in intensive agricultural areas are usually much greater in ground water than in surface water. The 1978-90 median dissolved-solids concentration was 533 mg/L and median nitrate was 12 mg/L for the shallow (less than 82 ft) Platte River alluvial aquifer [16]. Of the 12 pesticides listed in the table with median concentrations greater than the method detection limit, alachlor was less than 0.2 µg/L, atrazine was 0.3 µg/L, cyanazine was less than 0.1 µg/L, and metolachlor was less than 0.1 µg/L. These pesticide concentrations are generally greater than the medians of these compounds in surface water at the most upstream surface-water site at Tri-County Canal and less than the median concentrations at the most downstream surface-water site at Louisville.
Three locations in the Platte Valley were studied to assess the effects of water withdrawals on ground-water quality near the river [ 17 ]. The areas studied were a wet meadow area located about 15 miles east of Kearney, the city of Grand Island's Indian Island wellfield, and the city of Lincoln's Ashland wellfield.
The wet meadow is an area that has had very little ground-water development and represents essentially unstressed ground-water conditions. A number of observation wells were installed in the wet meadow at selected depths and distances downgradient from a corn field to study the areal and vertical differences in concentrations of selected water-quality constituents. The median dissolved-solids concentration of samples collected in February and June 1994 was 822 mg/L, somewhat above the medians for dissolved solids in surface water at sites upstream and downstream from the wet meadow site. The most pronounced changes in dissolved-solids concentrations occurred with depth. For example, the dissolved-solids concentration for December 1994 was 956 mg/L in the shallowest well in a cluster of wells completed closest to the corn field; the concentrations decreased to 575 mg/L in the deepest well of the cluster, completed at the bottom of the alluvial aquifer. The latter is nearly the same as the median dissolved-solids concentration in the streamflow just downstream at Grand Island. Nitrate concentrations in water samples from the shallow wells in the wet meadow in 1994 ranged from 5.0 to 15 mg/L. These concentrations were generally much larger than those detected in the associated deeper (30 and 45 ft) wells where nitrate concentrations ranged from 1.3 to 6.8 mg/L. Atrazine was detected in small concentrations of 0.1 to 0.6 µg/L in water from all of the wells sampled in February and June 1994, similar to the range of median atrazine concentrations detected in surface water at the upstream and middle sites. Concentrations of the other 11 herbicides were at or below the detection limit of 0.05 µg/L.
Grand Island is typical of cities in the Platte Valley. Concerns about adequate future supplies and the potential for ground-water contamination led to the development of the Indian Island wellfield site in the Platte River. It was anticipated that, with an island location, recharge from the river would act as a barrier to potential ground-water contamination of the wellfield and provide an additional source of good quality water to the aquifer. Control of land use within the wellfield also has the potential to minimize ground-water contamination from land-surface activities in the valley (Gary Mader, Utilities Director, City of Grand Island, written commun., 1994).
Observation wells were installed in four clusters of five wells each in the wellfield. An additional cluster of four observation wells was installed immediately north of the river. The wells in the wellfield clusters were completed at depths ranging from 20 to 123 ft, and wells in the cluster located north of the river were completed at depths of about 13 to 114 ft. Dissolved-solids concentrations varied with location and depth of the observation well. The concentrations ranged from 443 to 746 mg/L in the wellfield, with a median concentration of 578 mg/L. Because of controlled land use and the river barrier, the nitrate concentrations were very low. The median concentration of nitrate for samples collected during late 1994 and early 1995 was 1.37 mg/L. In late August to early September 1995, nitrate concentrations ranged from less than the detection limit of 0.05 mg/L to a maximum of 0.59 mg/L.
Water collected between December 1994 and September 1995 from the observation wells in the Grand Island wellfield also was analyzed for the same 12 pesticides shown in the table. Of the 12, only atrazine, prometon, and propazine were detected at concentrations of 0.05 µg/L or greater. Atrazine was detected at all five observation-well clusters but only in small concentrations. Atrazine was not detected in samples from any of the 123-ft-deep wells. At the cluster most affected by pumping, atrazine was detected in only the shallowest well. The maximum concentration of atrazine from the 47 samples collected at the five cluster sites was 0.38 µg/L. Prometon was detected in three and propazine was detected in two of 47 samples from the five well clusters. Maximum concentrations of prometon and propazine were 0.06 and 0.41 µg/L.
The Ashland wellfield provides almost all of the public supply water for the city of Lincoln, which has a population of about 200,000. Because ground-water withdrawals are large and the supply wells are near the river, major ion and nutrient concentrations in ground water are comparable to the Platte River water at the Louisville site. In studies conducted by others [ 18 , 19 ], atrazine detected in samples from the observation and supply wells were traced to the river. Atrazine concentrations in water sampled from observation wells in the wellfield were reported as high as 20 µg/L.
Inducing infiltration from the Platte River may have the effect of reducing nitrates to below that of the nearby ground water but also may have the effect of increasing pesticide concentrations. This is particularly true in the lower basin where the surface water may carry higher levels of pesticides and especially during the late spring and early summer when pesticide concentrations in the streams are at their highest. Generally, the closer a well is to a stream and the higher the pumpage rate, the more the quality of the well water will be influenced by the quality of the stream.
