IDENTIFYING SALINITY SOURCES IN THE NORTHERN SEGMENT OF THE BRAZOS RIVER ALLUVIUM AQUIFER

The Brazos River Alluvium aquifer is a minor aquifer in the State of Texas, and the northern segment of the aquifer includes Bosque, Hill, McLennan, and Falls counties. Variability in salinity can be seen in wells throughout the Brazos River Alluvium aquifer; however, the source of this variability is unclear. The objective of this study is to identify the sources of salinity in the northern segment of the Brazos River Alluvium aquifer. Potential sources of salinity include irrigation, an influx of water from the Brazos River, and contamination from historic oil and gas fields. For this study, salinity is defined as equivalent to total dissolved solids (TDS). An analysis of data from the Texas Water Development Board Groundwater Database shows TDS in the northern segment of the aquifer ranges from 347 mg/L to 3,040 mg/L, with a mean of 1,016 mg/L. Using a combination of water quality data and land-use maps, the effects of recharge and land-use over time (decades) on salinity in the aquifer, were evaluated. Hourly data on water level and conductivity from various data loggers in McLennan County were also used to investigate the variation in salinity over a much smaller time scale. TDS data for the Brazos River were obtained from three monitoring stations in the study area maintained by the Brazos River Authority. TDS in the northern segment of the river ranges from 158 mg/L to 2,040 mg/L, with a mean of 643 mg/L. The data show that average TDS of the aquifer is significantly higher than the average TDS of the river. Based on preliminary spatial and statistical analysis, an influx of water from the Brazos River or contamination from historic oil and gas fields are not likely the source of salinity within the northern segment of the Brazos River Alluvium aquifer.