SPATIAL VARIATIONS IN WATER CHEMISTRY AT SELECTED SPRINGS IN BIG BEND NATIONAL PARK

As part of a long term watershed study in the Pine Canyon area of Big Bend National Park, we have collected water samples from a number of springs and wells. The sample locations include Reynold’s well at an elevation of 1573 m, Chilicotal and Glenn Springs (elevations 853 m and 791 m, respectively) on the east side of the Chisos Mountains, and Oak Spring (elevation 1237 m) on the west side of the Chisos Mountains. The samples have been analyzed for the common cations and anions, alkalinity, pH, and conductivity. The data demonstrate expected increases in total dissolved solids (TDS) with decreasing elevation, ranging from 77.7 +/- 3.8 mg/L in the dilute water found at Reynold’s well (n=21) to 203.4 +/- 7.3 mg/L at Chilicotal Spring (n=12) and 583.6 +/- 55.2 mg/L at Glenn Spring (n=24). TDS at Oak Spring is intermediate (282.6 +/- 10.5 mg/L; n=10). This variation is primarily due to aquifer residence time, evaporation/precipitation effects, and possibly variations in aquifer geology; all are natural, inorganic, and non-anthropogenic processes. As demonstrated by the increase in TDS, the abundance of most analytes increases with decreasing elevation. Exceptions to this are the abundances of NO3 and SO4. NO3 and SO4 abundances are highest at Oak Spring (6.4 and 48 mg/L). Reynold’s Well has the second highest NO3 abundance (1.7 mg/L), but a relatively low SO4 abundance (11.9 mg/L). We believe that the elevated NO3 levels at Reynold’s Well can be explained by interaction of local precipitation with the abundant leaf litter accumulation in upper Pine Canyon, which is the recharge area for this well. The elevated NO3 and SO4 levels found at Oak Spring require an anthropogenic source. We speculate that the sewage treatment ponds in the Chisos basin might be this source. An important note is that all NO3 and SO4 values, including those from Oak Spring, are well under the EPA national primary or secondary maximum contaminant levels.