CONTROLS ON SURFACE WATER CHEMISTRY OF THE LAKE WACO DRAINAGE BASIN, WACO, TX

The surface water chemistry of the Lake Waco drainage basin has been studied intensively because of high nutrient loading to the lake. We have chemically and isotopically characterized baseflow to gain an understanding of the fluvial geochemistry of this drainage basin. The Lake Waco drainage basin is divided into four sub-basins each of which has unique groundwater chemistry. The groundwater flow systems, varying underlying lithologic units, and varying landuse can account for the chemical loads and compositions of the streams.

The largest of the sub-basins is the North Bosque watershed which contributes 75% of the water and 61% of the total dissolved solids (TDS) that flow into Lake Waco. Under baseflow conditions, the North Bosque contributes 91% of the orthophosphate load and 20% of the nitrate load. The Middle Bosque, South Bosque, and Hog Creek sub-basins, the other three sub-basins, contribute approximately 24%, 9%, and 8% of the TDS flowing into Lake Waco daily. The contrast in nutrient loads can be attributed to land use in the different basins. Loading of non-nutrients is mainly accomplished by the North Bosque River, which delivers 57% of the calcium, 79% of the magnesium, 69% of the sodium, 71% of the sulfate, 58% of the fluoride, and 64% of chloride. Loading of non-nutrients is strongly correlated to underlying rock type. For example, the high concentrations of Mg in the North Bosque are spatially related to outcrops of the dolomitized Glenn Rose Formation.

Oxygen and sulfur isotopic ratios measured in baseflow illustrate the differing contributions of shallow and deep ground water. The oxygen isotopic composition of baseflow in the North Bosque varies between –2.8 and –4.0 and is correlated to discharge. During high discharge, baseflow oxygen is light and derived from shallow groundwater flow systems. Heavier oxygen isotope ratios in occur during times of low discharge and represent groundwater derived from deeper parts of the aquifer. d³⁴S values for the North Bosque, Hog Creek, the Middle Bosque, and the South Bosque are –5.5, 3.4, 4.3, and –4.7 per mil respectively suggesting that shallow ground waters influence the baseflow of Hog Creek and the Middle Bosque whereas deeper groundwaters make up a volumetrically more significant part of the base flow in the other two streams.