Water quality of Lake Granbury, north-central Texas

Abstract

During water years 1970-79, the concentrations of the major dissolved constituents in Lake Granbury on the Brazos River in north-central Texas averaged about 1,800 milligrams per liter of dissolved solids, 700 milligrams per liter of chloride, and 350 milligrams per liter of sulfate. The water was generally very hard (hardness as calcium carbonate greater than 180 milligrams per liter). The concentrations of principal dissolved constituents varied throughout the year in relation to releases from Possum Kingdom Lake and to runoff from the intervening drainage area. Thermal stratification in Lake Granbury usually begins during April and persists until September or October. Stratification causes significant seasonal and areal variations in the concentration of dissolved oxygen, which in turn cause variations in the concentrations of dissolved iron and 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 30 to 40 feet (9 to 12 meters) usually contains less than 1.0 milligram per liter of dissolved oxygen. Variations in the concentrations of dissolved solids are associated with localized inflows. During periods of 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 De Cordova Bend Dam, iron concentrations in water near the bottom during summer stagnation ranged from 50 to 700 micrograms per liter and averaged 230 micrograms per liter. Manganese concentrations ranged from 1,300 to 3,700 micrograms per liter and averaged 1,800 micrograms per liter. During periods of winter circulation and in water near the surface during summer stagnation, both iron and manganese concentrations averaged less than I00 micrograms per liter. The concentrations of total inorganic nitrogen and total phosphorus are greatest during summer stagnation in water near the bottom at deep-water sites. At site A c during the summer, the concentration of total inorganic nitrogen averaged 2.37 milligrams per liter and the concentration of total phosphorus averaged 0.19 milligram per liter. The concentrations of both these constituents in waters near the surface during the entire year and in bottom waters during the winter averaged 0.07 milligram per liter or less. Seasonal-temperature variations and variations in the concentration of dissolved oxygen result in dissolved iron and manganese, total inorganic nitrogen, and total phosphorus being recycled within the lake; however, no significant accumulations of these constituents were detected.