|
|
Shift in the Relationship Between the Guadalupe River and the Trinity AquiferReferences: (a)
Analysis of Stream Flow
for the Guadalupe River Kerrville to Spring Branch - Summer Flow Rates 1951– 2000 (b) Effect of Rain the Week of Sept 13,
2000 on River Flow and Recharge Reference (a), shows why the residents of Comal County are concerned about the unregulated increase in developments and associated water use. While we can be assured that the population growth in the last 10 years puts a strain on water resources in years of drought, we also need to conduct studies to find out if this is the sole source of water depletion in the Trinity Aquifer. During the summers of 1996 and 2000, the Guadalupe River began to recharge the Trinity aquifer from Comfort to Spring Branch with water flowing out of the aquifer into the river above Kerrville. This is unprecedented and all contributing factors must be determined. In addition, instead of
drying up uniformly from Kerrville to Canyon Lake as in previous drought periods, the
river ran dry just below Hwy 311 while there was still ample flow at Kerrville and
Comfort. This is equally unprecedented. Concerning the pumping
restrictions of the Edwards Aquifer to preserve Comal Springs, at this time we must be
thankful for protection of the endangered species in the springs. Until the relationship between the Trinity
Aquifer and the Edwards Aquifer is better understood, the level of the Edwards must be
restricted to a high level. There
is a possibility that pulling down the water level in Edwards aquifer causes excessive
drainage of the Trinity aquifer. We all
know, based on the list of major water abusers that appeared in the Express-News November
20,2000, that given the choice of drying up wells in Comal County and watering their St.
Augustine grass, the residents of Bexar County will water the grass. Short
of anything else to control water use in Bexar County we must say, “all hail the minnow.” While the phenomenon we are seeing is
associated with drought conditions, the data presented in the reports show that the
aquifer normally, in the absence of rain, stays full for long periods and adds water to
the Guadalupe as you progress down stream. There
is usually more flow downstream than upstream as the Trinity Aquifer feeds the Guadalupe
River with spring water all along its length between Kerrville and Canyon Lake. As the water table drops from lack of rain, the
river dries up uniformly all along this length. However,
years 1996 and 2000 are different from any in the historical USGS record. The river is drying up downstream first. This can be explained only by water use, either by
direct pumping from the river, pumping from wells in the Trinity aquifer, or unusual
outfall from the Trinity Aquifer to the Edwards as the Edwards is being drawn down. If the trend indicated by
this data continues, the Guadalupe River will dry up much sooner in periods of drought
than it has historically. It
is unacceptable for the Guadalupe to dry up at the
Spring Branch gage when there is substantial flow at the Kerrville and Comfort gages, . Without this river there is no Hill Country. The data in reference (b),
shows a remarkable flow of water into the Trinity Aquifer after a recent rain. As noted in reference (a), the Guadalupe River had
been dry at Rebecca Creek Rd. Bridge for some weeks.
We personally visited the bridge every few days after noting a surge of
water at the USGS gage at Comfort on September 13. The
river was still dry at Rebecca Creek Rd. Bridge on Sept 16 whereas flow of 45 cfs was
recorded at Spring Branch gage, just 4.5 miles upstream, as early as Sept 14. In short, over 48 million gallons of
water was soaked up in a 40-hour period in this 4.5-mile stretch of the Guadalupe. As the Trinity absorbed all the water it could at
this rate, the additional water coming downstream after that date was forced down the
river and into Canyon Lake. The Trinity
Aquifer had to be severely depleted prior to this rain for this huge recharge to occur. The big question is where did the water go. Into Canyon Lake? The Edwards Aquifer feeding Comal Springs via a branch fissure in the Balcones Fault? Study is needed here. There is a known sink hole between the Spring Branch gage and Rebecca Creek Rd. bridge that takes all the water from the river during low flow periods. A USGS gage at Rebecca Creek Bridge and water well monitoring on the Trinity aquifer along the Guadalupe River and along the Balcones Fault would be a great start. The Balcones Fault is the boundary between the Trinity and the Edwards Aquifer and it is known that water flows from the Trinity to the Edwards. The quantity of water transferred is not known. Water well monitoring is tough because ideally wells that are not pumped are needed in order to monitor aquifer level. Our knowledge of the aquifer must be improved, In the interim, water use and development over the Trinity must be strictly regulated. Comal County is becoming
urbanized at a rapid rate. As more
developments go in people fail to realize that many established developments are not fully
populated as many people have purchased lots for retirement. The water companies are promising water to every
new development based on present capacity and without regard to empty lots that will
someday contain homes. As has
happened in other areas, these companies will run out of allotted water and there will be
a waiting list for meters. People
who have purchased their land for retirement will not be able to realize that dream as the
meters will be sold on a first come first served basis. In the mean time, ALL HAIL
THE MINNOW. This little fish may be the only
thing that keeps Comal County from running out of water.
We really need to get a handle on the relationship between the Guadalupe
River, the Trinity Aquifer and the Edwards Aquifer. Stephen C. Grigory, P.E. |
|
This site is maintained by email scgrigory@grigory.com for technical
comment or questions |