Water quality of Somerville Lake, south-central Texas

Somerville Lake in south-central Texas is a shallow lake, with a mean depth of 14 feet. The maximum depth of the submerged channel of Yegua Creek is usually less than 35 feet and in most areas of the lake the depth is less than 10 feet.

Several factors including thermal circulation resulting from the cooling of surface water, wind action, and the large inflow volume in realtion to the lake volume combine to keep the lake well mixed throughout the year. The oxygen concentrations remain high areally and at depth because of good circulation of lake waters during most of the year. Even in summer most bottom oxygen concentrations were in excess of 50 percent of saturation.

Due to year-round high percent oxygen saturation from surface to bottom in most parts of the lake, caused by the frequent periods of circulation that occur during all seasons, concentrations of dissolved iron, and manganese reamin low. Dissovled iron concentrations were less than 50 micrograms per liter and dissolved manganese concentrations were less than 40 micrograms per liter. The total inorganic nitrogen concentrations varied little throughout the lake. During the summer, concentrations were 0.01 milligram per liter at the surface to 0.02 milligram per liter at the bottom; and during the winter 0.11 milligram per liter at the surface and 0.10 milligram per liter at the bottom. Concentrations in the headwaters were about double those in the lake. Surface and bottom total phosphorus concentrations, during summer and winter averaged about the same, 0.04 and 0.06 milligram per liter throughout the lake, except in the headwaters where the concentrations were about double those in the lake.

Homogeneous or near homogeneous concentrations of total phosphorus and inorganic nitrogen can occur at any time of the year throughout the lake. Total phosphorus concentrations did not increase during the year or during the study period. On the other hand total inorganic nitrogen concentrations did show an annual cycle and were highest in the spring and lowest in late summer or fall. During periods of large releases of water, the more soluble total inorganic nitrogen was flushed from the lake.

The concentration of dissolved solids ranged from 139 to 292 milligrams per liter and averaged about 220 milligrams per liter. Dissolved chloride concentrations ranged from 20 to 68 milligrams per liter and averaged 43 milligrams per liter. Dissolved sulfate concentrations ranged from 30 to 130 milligrams per liter and averaged 63 milligrams per liter. The total hardness of the water ranged from 75 to 140 milligrams per liter, expressed as calcium carbonate, placing it in the moderately hard to hard (61 to 180 milligrams per liter) classification. The concentrations of principal dissolved constituents indicate that Somerville Lake is an excellent source of water for municipal, industrial, or agricultural use.