DISSOLVED METAL ABUNDANCES IN BAYOU WATERS FROM HARRIS COUNTY, TEXAS: A POSSIBLE MONITOR OF SUBSIDENCE IN THE HOUSTON-GALVESTON AREA

Water flowing through bayous in Harris County, Texas has a significant effluent component from water treatment plants.  Questions concerning the quality of Houston-area surface waters have led to an examination of their dissolved metal abundances.  Samples of water were collected from approximately 90 localities from 12 bayous and tributaries, the Houston Ship Channel (surface and deep water samples) and Galveston Bay, as well as groundwater, rainwater, and municipal tap water over a two year period.  Air and water temperatures and pH were measured on-site.  Samples from several localities were collected multiple times throughout the course of the study to observe seasonal fluctuations in chemistry.  In addition, samples were collected after appreciable rainfall to observe the effects of surface runoff.  Leaching experiments were also performed to evaluate the effect of soil components on the composition of bayou waters.

Aside from minor variations, bayou water compositions are fairly uniform.  Results of soil leaching experiments indicate that their compositions are largely unaffected by subsurface runoff.  Dissolved metal abundances were below the maximum contaminant levels for drinking water established by the EPA (www.epa.gov), with the exceptions of Pb and Tl.  Elevated Pb levels (>0.1 mg/L) were in samples collected after heavy rainfall, suggesting Pb input into the bayou through surface runoff.  One bayou that flows predominantly through agricultural land had higher phosphorus contents (up to 6 mg/L) than the others.

The Houston-Galveston area has suffered for nearly a century from extensive subsidence caused by groundwater and oil extraction.  Present subsidence rates are estimated to be ~5 cm/yr in some areas.  The uniformity in bayou water compositions, the great difference in composition between bayou water and marine water in Galveston Bay, and the fact that influx of marine water has been detected as far as 40 km upstream during high tide suggest that the dissolved metal load in Harris County bayou waters may be a sensitive monitor of coastal subsidence, assuming that tidal fluctuations and variations in stream channel cross-sections can be accounted for.