LOCATING A SUSTAINABLE SMALL PUBLIC WATER SUPPLY IN A CONSTRAINED RESOURCE AREA OF DAKOTA AQUIFER, NEBRASKA

Inadequate supply of good quality ground water is a growth constraint for satellite communities of the Lincoln, Nebraska urban area. Availability of good quality water is limited by the natural occurrence of saline ground water in the Dakota Aquifer. The potential for intrusion of saline water from depths prohibits high capacity wells from being developed in some areas where fresh water occurs. Nitrate from agricultural practices impairs many shallow fresh aquifer areas. Water quantity is limited due to the low permeable areas of the Dakota and glacial drift aquifers. Also, there is public concern from acreage owners about the effects of new aquifer development on their existing domestic supply wells.

A study was initiated to locate a new wellfield for a small public water supply system in this area of constrained availability. Two existing shallow wells for the village of Emerald produce only 15 gpm each. They are impacted by drought, impaired by nitrate >10 ppm, and constrained by underlying saline water. A potential new site was identified using geologic mapping, land use maps and water quality sampling of area wells. Results indicated a fresh water recharge area characterized by low TDS, low nitrate and calcium bicarbonate type ground water that coincides with a bedrock-high of the Dakota Formation and an area of low row crop use.

Subsequent test drilling and geophysical logging showed a shallow unconfined, sandstone aquifer of the Dakota Formation. A 60 foot confining layer of mudstone separates it from deeper sandstone. Two test wells were completed in the shallow aquifer and one in the deep. Quality of the shallow aquifer is a fresh (<500 TDS) calcium bicarbonate type with low nitrate (<3 ppm). It is also lacking in iron and manganese, which are usually a problem in eastern Nebraska. Oxygen and hydrogen isotope data indicate that water from shallow wells represent modern recharge from meteoric waters. The deep confined sandstone is a brackish (TDS=2700) sodium-chloride water type. For the shallow aquifer, a hydraulic conductivity of 17.7 ft/day and a specific yield of 0.04 were determined from a 24 hour pumping test. Water quality samples taken at end of test showed no increase in salinity. Test data indicate two new wells would double the existing supply without interfering with nearby domestic wells.