Land-surface subsidence has been mapped in the Houston-Galveston area and is known to have occurred in other areas within the Texas coastal region. Most of the subsidence has been caused by both the withdrawal of ground water and by the production of oil, gas, and associated ground water. LAND-SURFACE SUBSIDENCE EXPLANATION 042 O LOCATION OF BENCH MARK WITH MAXIMUM TIME MECOn-NHmbw it decline I* •motto* totott. far period of record. 1*17-SI » LOCATION OF BENCH MARK WITH MIXED TIME MECORO THAT HAS A NET DECLINE IN ELEVATION MT| BENCHMARK BENTIFKATON Q SELECTED AREA OF CONCENTRATION OF WATER WELLS
The hypothesis that patterns and modes of soil moisture migration can be interpreted from relationships between soluble sodium percentage (SSP) and total soluble cations (TSC) with depth in soil profiles is developed. Capillary migration of moisture during the soil drying process is more apt to leave a chemical imprint on soil profiles than the initial wetting process since it occurs last, and cations move in closer proximity to exchange surfaces. Soil profiles must be sampled in contiguous increments small enough to differentiate important variables. Each soil sample must be wetted exactly to saturation prior to extraction of soluble Ca, Mg, and Na for analysis. Both cation exchange and salt precipitation cause progressive losses of multivalent cations from solution as moisture moves through soil. Resulting increases in SSP indicate directions of moisture migration. Decreases in TSC accompanied by increases in SSP apparently result from movement in thin films in response to capillary tensions > 1 bar (salt sieving). Moisture migration in extremely thin films in response to high tensions at forward extremities of capillary migrations apparently produces increases in TSC values accompanied by increases in SSP values (salt precipitation).
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