Hydrological Barriers Research Articles

Fault Control of Subsidence, Houston, Texasa

ABSTRACTLand subsidence increases the area in the Texas Coastal Zone which will be inundated by marine waters from hurricane flooding. Storm surge from a Carla‐sized hurricane in 1976 would flood at least 25 square miles (65 km2) more land than Hurricane Carla did in 1961.Land subsidence in Harris and Galveston Counties results primarily from ground‐water production. The two‐county area is interlaced with active surface faults with topographic escarpments and surface faults with no topographic escarpments that control drainage patterns and create subtle photographic linear patterns. Ground‐water production activates these faults by differential compaction of the aquifer. The faults appear to be partial hydrologic barriers that compartmentalize land subsidence into several individual basins.

Tracer aids interpretation of pumping test

The nuclear research center at Mol, Belgium, is underlain by clean quartz sand to a depth of about 30 meters. An accident at one of the nuclear facilities, which could contaminate the ground water, would entail serious consequences, since the movement of radioactive contaminants through the ground would be retarded or reduced only slightly by adsorption. Test drilling showed that the water table aquifer is composed of two sand formations, the upper Mol sand being somewhat coarser grained and therefore probably more permeable than the underlying Casterle sand. The lower part of the Mol sand contains a number of thin clay layers which make a partial hydrologic barrier between the two formations, dividing the water table aquifer into two parts. A test well was installed, screened through the full thickness of both sands, and test pumped. Drawdowns were measured in observation wells screened both in the Mol sand and in the Casterle sand. A tracer was added to four of the observation wells in turn, and the arrival of the tracer was measured in the pumped well. The average travel times in the Mol and Casterle sand were used to determine the amount of water being produced from each by the pumping, and these data, with the observed drawdowns, were used to calculate the transmissibility, the permeability, and the storage coefficient of each of the two formations. Using these parameters, the designs of several well systems for pumping contaminated ground water out of the aquifers are discussed. Pumping out, or scavenging, the contaminated ground water and decontaminating it appears to be a practical method of resolving some of the problems that would be raised by a nuclear accident in the Mol area.

Formula omitted] disequilibrium as an aid to hydrologic study of the Floridan aquifer

Investigation of the distribution and environmental disequilibrium patterns of naturally occurring uranium isotopes ( 234U, 238U) in waters of the Floridan aquifer in north Florida, utilizing the analytical methods of isotope dilution and alpha-particle spectrometry, indicates that variations in isotopic activity ratios and concentrations can be related to the regional hydrogeologic framework. The data permit interpretation in terms of (1) regional permeability characteristics, including the existence of hydrologic barriers, (2) groundwater circulation patterns, (3) sources of waters from various parts of the hydrologic environment, and (4) areas of extensive Pleistocene leaching within the Floridan aquifer.

INFLUENCES OF ABSOLUTE AND PARTIAL HYDROLOGIC BARRIERS ON PUMP TEST RESULTS

A review is presented of the method of images as used in situations in which there is an absolute hydrologic barrier, and possible sources for misinterpretation of pump test results are stressed. The method is then applied to the case of non-equilibrium flow in the presence of a partial hydrologic barrier, under certain limitations. It concluded that the presence of such a barrier may not be reflected in any discernible way in drawdown–time curves resulting from pump tests, or, if it is reflected, may easily be overlooked.