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Water in Lake Arlington on Village Creek in north-central Texas had volume-weighted average concentrations of less than 240 milligrams per liter of dissolved solids, less than 30 milligrams per liter of dissolved chloride, and less than 40 milligrams per liter of dissolved sulfate between January 29, 1973, and August 20, 1981. The water was moderately hard (hardness greater than 60 but less than 120 milligrams per liter as calcium carbonate). The concentrations of each of these constituents were usually greatest during winter, especially during the first few years of the study, but decreased significantly as discharges of municipal wastes were systematically and progressively diverted to treatment facilities downstream from the reservoir. Thermal stratification in Lake Arlington usually begins during March or April and persists until October. Thermal stratification has resulted in significant seasonal and areal variations in the concentration of dissolved oxygen, dissolved iron, dissolved manganese, total inorganic nitrogen, and total phosphorus. Oxygen utilized in the decay of organic matter is not replenished during periods of summer stagnation, and water below depths of 30 to 40 feet usually contains less than 2 milligrams per liter of dissolved oxygen during stagnation. Even though heated effluent from an electrical generating plant is returned to the reservoir at site GC and causes an elevation of water temperature, average dissolved oxygen levels at this site are not significantly different from levels at other sites. During summer stagnation, reducing conditions result in the dissolution of iron and manganese from bottom deposits at deep sites in the reservoir. At site AQ, a deep site near Arlington Dam, dissolved iron concentrations in water near the bottom during summer stagnation ranged from less than 10 to 1,100 micrograms per liter and averaged about 640 micrograms per liter. Dissolved manganese concentrations near the bottom at site AC during summer staynation ranged from 20 to 2,700 micrograms per liter and averaged about 1,500 micrograms per liter. The concentrations of dissolved iron and dissolved manganese in water throughout the reservoir during winter circulation and in water near the reservoir surface during summer stagnation averaged less than 50 micrograms per liter. Seasonal temperature and dissolved oxygen cycles resulted in the recycling of dissolved iron and dissolved manganese between the water and bottom sediments. However, no significant accumulation of these constituents within the reservoir was detected during the study. The concentrations of total inorganic nitrogen and total phosphorus are greatest during summer stagnation in water near the bottom at deep sites. At site AC during the summer, the concentrations of total inorganic nitrogen in the hypolimnion averaged about 0.9 milligram per liter, and the concentration of total phosphorus near the bottom averaged about 0.2 milligram per liter. The concentrations of total inorganic nitrogen in the epilimnion at site AC averaged about...

Thermal stratification in Lake Conroe usually begins to develop in March and persists until October. Thermal stratification has resulted in significant seasonal and area! variations in the concentrations of dissolved oxygen, dissolved iron, dissolved manganese, total inorganic nitrogen, and total phosphorus. Volume-weighted-average concentration of dissolved solids generally were less than 120 milligrams per liter, those of dissolved chloride generally were less than 22 milligrams per liter, and those of dissolved sulfate were less than 10 milligrams per liter in Lake Conroe during the 1973-82 water years. The concentrations of each of these constituents usually were largest during the summer. The water was moderately hard (hardness greater than 60 but less than 120 milligrams per liter as calcium carbonate). The average concentrations of dissolved oxygen at most sites in the downstream one-half of the lake averaged 3.2 milligrams per liter during summer stratification and more than 9 milligrams per liter during winter circulation. The concentrations at most sites in the headwaters of the lake averaged less than 4.3 milligrams per liter during the summer and less than 7.9 milligrams per liter during the winter. Water below depths of 25 to 35 feet usually contained less than 1 milligram per liter dissolved oxygen during the summer. The concentrations of dissolved iron and dissolved manganese in water throughout the reservoir during winter circulation and in water near the reservoir surface during summer stratification were less than 100 micrograms per liter. The greatest concentration occurred during summer stagnation near the reservoir bottom at site Ac , a deep site near Lake Conroe Dam. The concentrations of total inorganic nitrogen and total phosphorus were greatest during summer stratification in water near the reservoir bottom at deep sites. No accumulation of these constituents within the reservoir was detected during the study. The densities and composition of algal populations varied seasonally. Algal densities were greatest during the summer with blue-green algae being the predominant phylum. I2 13 concentrations of dissolved winter surveys.

Significant upward trends in dissolved-solids concentrations were detected with the Seasonal Kendall Test for trends at three stations in the upper basin during the study period. The increases exceeded 270 milligrams per liter per year at two stations and 165 milligrams per liter per year at the third station. The composition of dissolved constituents in the Colorado River basin changes from predominantly sodium and chloride ions in the upper basin to predominantly calcium and bicarbonate ions in the lower basin. The U.S. Environmental Protection Agency secondary drinking-water regulations of 500 milligrams per liter for total dissolved solids was exceeded 95 percent of the time at each station on the main stem of the Colorado River in the upper basin. In the middle Colorado River basin, the Environmental Protection Agency secondary drinking-water regulations for total dissolved solids was exceeded approximately 95 percent of the time at most stations. Nutrient concentrations in the Colorado River basin generally were low. Only one sample exceeded the level set for nitrate nitrogen, and no other nutrient species exceeded Environmental Protection Agency levels. A general upward trend was detected in organic nitrogen and total nitrogen, but concentrations still remained low. Densities of fecal-col iform and fecal-streptococcal bacteria ranged from less than 1 colony per 100 milliliters to 26,000 colonies per 100 milliliters and 1 colony per 100 milliliters to 50,000 colonies per 100 milliliters, respectively. Fecal-coliform densities exceeded Environmental Protection Agency criteria for public water supply (2,000 colonies per 100 milliliters) at several stations during the study. Biochemical oxygen demand concentrations ranged from 0.00 to 34 milligrams per liter. Only one mean biochemical oxygen demand concentration exceeded 8 milligrams per liter, the upper range of concentration common in moderately contaminated streams. Trace elements and pesticides were detected in many samples throughout the basin. The concentrations generally were low, and maximum contaminant levels rarely were exceeded.

Lake Granbury is owned and operated by the Brazos River Authority for the conservation of water for industrial use, irrigation, and municipal supply. Since 1970, an average of approximately 22,700 acre-feet (28 hm 3) of water has been withdrawn annually for these purposes. Approximately 66 percent of the water is withdrawn for industrial use and approximately 34 percent for irrigation. Less than 1 percent is withdrawn for municipal supply. Metric Conversions Factors for converting inch-pound units to metric equivalents are given in the following table: Multiply From by To obtain acre 4,047 square meter (in 2) acre-foot 0.001 233 cubic hectometer (hm 3) cubic foot per second 0.02832 cubic meter per second (m 3 /s) foot 0.3048 meter (in) mile 1 .609 kilometer (kin)