Water quality of Lake Whitney, north-central Texas

The volume-weighted average concentrations of the major dissolved constituents in Lake Whitney on the Brazos River in north-central Texas usually were less than 1,300 milligrams per liter of dissolved solids, 500 milligrams per liter of chloride, and 300 milligrams per liter of sulfate during the 1970-80 water years. The water was very hard (hardness greater than 180 milligrams per liter as calcium carbonate). The concentrations of principal dissolved constituents varied in relation to releases from Lake Granbury and in relation to runoff from the intervening drainage area. Releases from upstream reservoirs associated with heavy rainfall during August 1978 showed a marked increase in concentrations of all dissolved constituents.

Thermal stratification in Lake Whitney usually begins during April and persists until October. Stratification causes significant seasonal and areal variations in the concentration of dissolved oxygen, which cause variations in the concentrations of dissolved iron, dissolved manganese, total inorganic nitrogen, and total phosphorus. Oxygen utilized in the decay of organic matter and bottom material is not replenished during periods of summer stagnation, and water below depths of 40 to 50 feet (12 to 15 meters) usually contains less than 2.0 milligrams per liter of dissolved oxygen.

During summer stagnation, reducing conditions result in the dissolution of iron and manganese from the bottom deposits in the lake. At site AC, a deep site near Whitney Darn, dissolved iron concentrations in water near the bottom during summer stagnation ranged from 90 to 400 micrograms per liter and averaged about 195 micrograms per liter. Dissolved manganese concentrations ranged from 240 to 2,100 micrograms per liter and averaged about 1,400 micrograms per liter. During winter circulation and in water near the surface during summer stagnation, both iron and manganese concentrations averaged less than 40 micrograms per liter.

The concentrations of total inorganic nitrogen and total phosphorus are greatest during summer stagnation in water near the bottom at deep sites. At site A c during the summer, the concentration of total inorganic nitrogen near the bottom averaged 1.3 milligrams per liter and the concentration of total phosphorus near the bottom averaged 0.28 milligram per liter. During the entire year, the concentration of both these constituents in water near the surface averaged less than 0.05 milligram per liter, with the exception of site P12.

Seasonal temperature variations and variations in the concentration of dissolved oxygen result in dissolved iron, dissolved manganese, total inorganic nitrogen, and total phosphorus being recycled within the lake; however, no significant accumulations of these constituents were detected.