Major and trace element hydrochemistry in a spring-fed river (Spring River, Arkansas)

This study assesses the unique hydrology of the Spring River of Arkansas and the consequent variations in the chemistry of the waters. The Spring River supports one of the most diverse fish fauna in the south-central region of the U.S. and is an economic base for this region of Arkansas. The mouth of the Spring River in northeast Arkansas is located at Mammoth Spring where 34 million liters of water per hour flow out of the spring mouth at a constant temperature of ~ 150 C. The spring water is discharged from a limestone-dolostone aquifer. This slightly alkaline high Ca water "warm" end-member (zone 1) mixes with cooler downstream waters derived primarily from surface run-off. Groundwater discharge into the trunk and tributary stream beds lessens with distance from the spring. The chemistry of the waters, particularly the temperature and major solute chemistry demonstrates simple binary to ternary mixing of water sources. Along the length of the river three distinct temperature zones develop and persist year round. Zone 1 is located close to Mammoth Spring and is characterized by constant year round temperatures of 14-degress to 15-degrees C. Approximately 20 km downstream tributaries intersect the trunk stream creating a mixed zone (zone 2). Approximately 50 km down stream of the mouth the water temperature reaches its lowest average year-round temperature (zone 3). The major and trace element chemistry of the zones are unique and therefore serve as end-member compositions for mixing models. Of particular interest to this study are the episodically high levels of nitrate in the spring water (~20-30 mg/L). There is a seasonally defined trend of nitrate concentrations decreasing with distance from the spring mouth. Superimposed on this trend are recurrent high nitrate levels derived from small tributaries approximately 20-30 km down stream from the spring. Early studies associated the high nitrate levels with effluent from the State and Federal Hatcheries located downstream from the spring mouth however our results do not support the conclusion that these industries are nitrate point sources. Rather there appears to be two discrete nitrate sources, spring water and agricultural run-off into smaller tributaries (ditches). As part of this study we are assessing the feasibility that the nitrate in the spring water is geologic and is derived from the aquifer material.