Chemical Classification and Trophic Characteristics of Texas Reservoirs

ABSTRACT

Texas reservoirs were classified into three groups by cluster analysis based upon the following properties of their surface waters: specific conductance, alkalinity, calcium, sulfate, pH, chloride, and sodium (listed in descending order of importance as discriminators). The three groups of reservoirs had a distinct geographic distribution corresponding to a strong east-west gradient of increasing aridity and differences in surface geology. Chemical characteristics of the western reservoirs were most influenced by the dissolution of evaporites formed in the geological past and current arid conditions (median specific conductance = 1817 μS cm⁻¹; median alkalinity = 120 mg CaCO₃ L⁻¹). Sulfate and chloride accounted for 87% of the major anions (in equivalent charge concentrations). The second group of reservoirs, located in a wide band through central Texas, were dominated by weathering of limestone bedrock and calcareous soils (median specific conductance = 518 μS cm⁻¹; median alkalinity = 133 mg CaCO₃ L⁻¹). Alkalinity accounted for 53% of the anions. The eastern group, generally located on sedimentary muds and clays east of the Balcones/Ouachita trend, receive much higher runoff than other areas of the state. They were more dilute (median specific conductance = 185 μS cm⁻¹; median alkalinity = 29 mg CaCO₃ L⁻¹), and alkalinity accounted for 45% of the anions. The central group had the highest proportion of oligotrophic reservoirs (30%) based on near-dam, growing season chlorophyll a concentrations, and also had the lowest concentrations of total phosphorus. Western and central reservoirs tended to be deeper with proportionally much smaller littoral zones, and precipitation of CaCO₃ may be quite important in moderating the productivity of these impoundments, relative to reservoirs in eastern Texas.

Key Words:

• reservoirs
• Texas
• nutrients
• climate
• geology
• alkalinity
• chlorophyll
• ionic composition