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In Nebraska, a large number (>200) of shallow sand-point and cased wells completed in coarse alluvial sediments along rivers and lakes still are used to obtain drinking water for human consumption, even though construction of sand-point wells for consumptive uses has been banned since 1987. The quality of water from shallow domestic wells potentially vulnerable to seepage from septic systems was evaluated by analyzing for the presence of tracers and multiple isotopes. Samples were collected from 26 sand-point and perforated, cased domestic wells and were analyzed for bacteria, coliphages, nitrogen species, nitrogen and boron isotopes, dissolved organic carbon (DOC), prescription and nonprescription drugs, or organic waste water contaminants. At least 13 of the 26 domestic well samples showed some evidence of septic system effects based on the results of several tracers including DOC, coliphages, NH4+, NO3−, N2,δ15N[NO3−] and boron isotopes, and antibiotics and other drugs. Sand-point wells within 30 m of a septic system and<14 m deep in a shallow, thin aquifer had the most tracers detected and the highest values, indicating the greatest vulnerability to contamination from septic waste. [ABSTRACT FROM AUTHOR]

During spring runoff events, herbicides in the Platte River are transported through an alluvial aquifer into collector wells located on an island in the river in 6 to 7 d. During two spring runoff events in1995 and 1996, atrazine [2-chloro-4-ethylamino-6-isopropylaminos-triazinel concentrations in water from these wells reached approximately7 mug/L, 70 times more than the background concentration in ground water. Concentrations of herbicides and metabolites in the collector wells generally were one-half to one-fifth the concentrations of herbicides in the river for atrazine, alachlor [2-chloro-2'-6'-diethylN-(methoxymethyl)-acetanifi del, alachlor ethane-sulfonic add (ESA) [2-((2,6-diethylphenyt) (methoxymethyl)amino)-2-oxoethane-sulfonic acidl, metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy- l-methylethyl)acetamidel, cyanazine [2-((4-chloro-6-(ethylamino)-1,3,5 triazin-2-yl)-amino)-2-methylpropionitrilel, and acetochlor [2-chloro-N-(ethoxymethyl)-N-(2-ethyl-6methyl-phen yl) acetamidel, suggesting that20 to 50% river water could be present in the water from the collector wells, assuming no degradation. The effect of the river on the quality of water from the collector wells can be reduced through selective management of horizontal laterals of the collector wells. The quality of the water from the collector wells is dependent on the (i) selection of the collector well used, (ii) number and selection of laterals used, (iii) chemical characteristics of the contaminant, and (iv)relative mixing of the Platte River and a major upstream tributary. [ABSTRACT FROM AUTHOR]

Water-supply managers need adequate warning to protect water supplies if a contaminant is spilled in an upgradient tributary. The city ofLincoln draws water from alluvium associated with the Platte River near Ashland, eastern Nebraska. Using constant-rate injection methods and a conservative tracer, travel time and degree of mixing of contaminants in the Elkhorn and Platte Rivers were evaluated in 1995 and 1996. The results indicate that, for flows of 584 to 162 m3sin the Platte River at Ashland with 13 to 28% of its flow contributed by the Elkhorn River, 8.2 to 13.2 h are required for the leading edge of a chemical plume to travel from the Elkhorn River at Waterloo to the Platte River at Ashland. The peak concentration of a chemical spilled as a slug in the Elkhorn River near Waterloo would pass the well field after 11.3 to 16.1 h. Existing empirical equations for calculation of travel time were shown to apply to reaches of streams studied, but underestimated the leading edge up to 14% and overestimated the plateau concentration up to 11% at Site 5. However, time of travelmay be influenced by the relative contribution of a tributary. The plateau concentration of the chemical in the Platte River at Ashland was 45 to 60% of its concentration in the Elkhorn River. The degree ofmixing of the tracer in the Platte River at Ashland increased from 53 to 65% as the relative contribution of the Elkhorn River increased. [ABSTRACT FROM AUTHOR]