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 0.1 milligram per liter; the concentrations of total phosphorus averaged less than 0.1 milligram per liter.

The densities and composition of algal populations varied seasonally. At site AC, total algae counts ranged from 200 to 240,000 cells per mi Hi liter and averaged about 50,000 cells per milliliter. At site FC, total algae counts ranged from 1,000 to 290,000 cells per milliliter and averaged about 56,000 cells per milliliter. Algal densities were greatest during the summer with blue-green algae being the predominant phyla.