Evaluation of future base-flow water-quality conditions in the Hillsborough River, Florida

Abstract

A one-dimensional, steady-state, uniform water-quality model was developed for a 30.0-mile reach of the Hillsborough River to simulate water-quality conditions expected from future development. The model was calibrated and verified using data collected under critical base-flow conditions in April and December 1978. Dissolved organic nitrogen, nitrate nitrogen, and total and fecal coliform bacteria were modeled for most of the study reach. Results from the model were used to evaluate the impacts of two typical housing developments on water-quality conditions in the Tampa Reservoir. One development was in the Cypress Creek basin, one of the major tributaries in the lower part of the study area, and the other was near the upper end of the reach of the Hillsborough River. The model analysis indicated that development in the Hillsborough River basin may cause high total and fecal coliform bacteria conditions. Simulated total coliform bacteria at the Tampa water-treatment plant for 1, 3, and 5 square-mile developments in the Cypress Creek basin were 3,000, 5,400, and 8,300 colonies per 100 milliliters. Similar developments located near the upper end of the study reach were 2,000, 3,600, and 5,100 colonies per 100 milliliters.. Simulated fecal coliform bacteria were 360, 700, and 100 and 180, 350, and 510 colonies per 100 milliliters, respectively. Other constituents modeled showed only minor increases in concentrations. A two-phase investigation of the Hillsborough River was initiated by the city of Tampa in cooperation with the U.S. Geological Survey in 1975. The purpose of the first phase of the study was to quantitatively evaluate the watersupply potential of the lower Hillsborough River, including the Tampa Reservoir, under existing conditions. Results of the first study phase are described in a report by Goetz and others (1978). The purpose of the second-phase study, which is described in this report, is to evaluate (using modeling techniques) water-quality characteristics of the basin under possible future conditions. This study phase involved collection of data to calibrate and verify (testing for acceptance within a specified error range) a water-quality model for a reach of the Hillsborough River that includes the Tampa Reservoir. The model is applicable during critical base-flow periods when concentrations of various constituents are highest. The U.S. Geological Survey one-dimensional, steady-state, uniform water-quality model (Bauer and others, 1979) was used. The purpose of the study is to apply a calibrated and verified model to simulate selected water-quality conditions that result from base-flow discharges from storm sewers for various sized residential developments. Results of the study estimate possible changes in water-quality conditions that may occur in the study reach as future development and stream-waste loadings from storm sewers increase. The model identifies only those changes in stream water quality that occur as a result of groundwater (base flow) infiltration into the storm-sewage system (s...