The Houston area receives more annual rainfall than nearly all of the other large cities in the State of Texas. With nearly 50 inches of rain a year as the norm, you would think that, since our rivers, streams, and bayous flood frequently, some of that extra runoff would make its way to the aquifers in the region. Unfortunately, that is not the case…to understand why that is, we have to discuss the hydrogeology, surficial geology, and where our groundwater comes from.
The Gulf Coast aquifer has been the primary source of water in the region for over a century. The aquifer extends throughout the Texas Gulf Coast. In the Houston region, the aquifer thickens from near Brenham and Navasota where it is very thin to the Gulf Coast where it can be many thousands of feet thick. It is comprised of many discontinuous lenses of sand, silt, and clay. The aquifer is prolific, in that it contains an immense supply of good quality water, with one very big “catch” – when you withdraw water from the aquifer and water-levels decline, the clay lenses in the aquifer compact and cause the land surface elevation to decrease (subsidence). In a flat, coastal, landscape, subsidence can contribute to flooding, property damage, and other serious consequences.
Throughout most of the Houston area, the aquifer material nearest the surface is predominately clay. This is great if you want to protect the aquifer from surface contaminants like pesticides and solvents which can cause serious groundwater quality problems. However, this nice protective clay “cap” also prevents storm water runoff from infiltrating the system. When you consider the natural geology, in addition to the amount of impervious cover (concrete) that extends across the Houston area, the amount of water that can make it through to the aquifer becomes very small. Recent studies by the USGS (USGS Scientific Investigations Report 2013-5024) show that the estimated age of groundwater in the most widely used portion of the aquifer system, the Evangeline aquifer, can exceed 40,000 years (an interesting thought isn’t it? The water you’re drinking could have landed on the surface of this area as rainfall more than 40,000 years ago!). Compare that to the Edwards aquifer (the primary source of water for the city of San Antonio) which is a limestone aquifer that exhibits much different physical characteristics, and has a simulated age of water between 1 and 342 years (USGS Fact Sheet 2011-3142).
Water use is the primary driver for water level in the Gulf Coast aquifer in Houston. This past spring, the Houston area received a considerable amount of rain, over 30 inches from around April 1 to July 1 (Figure 1). However, the water level in the most widely used portion of the aquifer only rose about 7 feet during that same time (Figure 2). And, although there does seem to be an empirical relation between rainfall and water level shown here, it is more likely that water use was substantially reduced due to the flooding and lack of irrigation water needs. Since the “dryout” began early in July, water level in the aquifer has begun to decline. Using the water level from Figure 2 as an example, any rise associated with the rainfall and lack of water use from April to July has been negated by the increased water use since the beginning of July to do the lack of rainfall area.
Sustainability of aquifers in the Houston area is the only way we can insure a viable resource in the future that can be utilized without the consequence of subsidence. 50 inches or more of rain a year (or 30 inches in 4 months) will not achieve our goals. Rather, a concerted effort by all of us to utilize our resources wisely and efficiently is needed.
Michael J. Turco is the General Manager for the Harris-Galveston Subsidence District. The Harris-Galveston Subsidence District is a special purpose district created by the Texas Legislature in 1975 to regulate groundwater withdrawal throughout Harris and Galveston counties for the purpose of preventing land subsidence.