An assessment of point and nonpoint pollution loads into Lake Houston
Newell, Charles J.
Bedient, Philip B.
Master of Science
The urbanization process has affected the characteristics of pollutant loads from both stormwater runoff (or nonpoint sources) and from sewage treatment plants (or point sources) in the tributaties of the 2828 mi lake Houston watershed. Land use projections indicate the urban development, which has caused water quality deterioration in seme tributaties, will continue at nearly the same overall rate in the 198 to 199 period as in the 197 to 198 period. The patterns of growth, however, will shift from development concentrated in the Cypress Creek and Lake Environs areas to several of the previously undeveloped watersheds; this will impact on water quality of the streams in these areas. A detailed point source analysis indicated that most of the larger sewage treatment plants acconmodating the increases in population in the Cypress Creek watershed had difficulty in providing accepted levels of treatment. All violated State of Texas standards for effluent quality to some degree, and treatment process upsets greatly increased effluent loads for sane parameters. The fate of the plant's nutrient loads at low flow was also examined: almost all of the nutrients were carried to the lake by the Cypress Creek low flow transport process. During these low flow periods, critical for lake water quality, point sources accounted for 25% of the nitrate-nitrogen (NOg) and 75% of the total phosphorus (TP) loads to Lake Houston. The nonpoint source analysis showed that urban development has increased total suspended solids (TSS) loads to lake Efouston. The rapidly urbanizing watershed, Cypress Creek, contributes 27-35% of the annual TSS load to the lake although it accounts for only 11% of the entire basin's area. The load analysis provided the following yearly pollutant loads to the lake (for an average year's flow under present development conditions): 159 • 16Kg TSS/y ear,31.8 * 1° Kg N^NO^/year, and 1.4 • 1 Kg TP/year. Because of continuing development in the entire basin, lake TSS loads are expected to rise to 194 • 1 Kg TSS/year in 199, a 22% increase. Two different basin-wide management policies were examined and results indicated that although stormwater detention sedimentation ponds were an effective measure to prevent this increase in TSS loads, they probably could not be applied to reduce TSS loads much below present levels. The instantaneous load-runoff methodology using periodic grab sample data, used to calculate nonpoint pollutant loads, seemed to work well for TSS loads in general and for nutrient loads in the undeveloped watersheds. The method seemed to underestimate nonpoint nutient loads from developing watersheds, probably because of the high variability of stormwater runoff concentrations in these areas. An intensive storm sampling program on two storms, used in the management policy analysis, was also applied to the load-runoff methodology. This analysis indicated that future studies designing monitoring programs to calculate annual pollutant budgets to Lake Kbuston should employ intensive storm sampling techniques rather than the sampling at regular intervals performed by earlier studies.