Dolomite dissolution rates and possible Holocene dedolomitization of water-bearing units in the Edwards aquifer, south-central Texas

Abstract

Rates of dolomite dissolution can be used to test the concept, based on geomorphologic evidence, that a major part of the Edwards aquifer could have formed within the Holocene, a timeframe of approximately 10,000 years. During formation of the aquifer in the Edwards limestone (Cretaceous, Albian) of the Balcones fault zone, dolomite dissolution and porosity development were synchronous and the result of mixing-zone dedolomitization. Initiation of the mixing zone in the early Holocene (∼11,000 years before present) is suggested by the maximum age of formation of major discharge sites that allowed the influx of meteoric water into brine-filled, dolomitic preaquifer units. Dedolomitization, the dissolution of dolomite and net precipitation of calcite, has left aquifer units that are calcitic, and 40 vol.% interconnected pore space. The mass of dolomite missing is obtained by comparison of stratigraphically equivalent altered and unaltered units. One dissolution rate (1.76 × 10 −4 mmol dolomite kg H2O −1 yr −1 ) is determined from this mass, 10 4 yr reaction time, and a log-linear function describing the increase in mass discharge (three orders of magnitude) during aquifer formation. The second estimated dissolution rate is obtained from the mass transfer of dolomite to solution calculated from the increase in magnesium in pore fluids selected from the modern aquifer to represent a typical flowpath during aquifer formation. A reaction time of 10 4 yr for this mass transfer yields a rate of 0.56 × 10 −4 mmol dolomite kg H2O −1 yr −1 Both of these rates are comparable to modern rates of dolomite dissolution (0.3 to 4.5 × 10 −4 mmol dolomite kg H2O −1 yr −1 ) calculated from measured reaction times in the Tertiary Floridan aquifer system in Florida and the Madison aquifer in the Mississippian Madison Limestone of the Northern Great Plains. Similarity of these rates to the estimated paleo-rates of dolomite dissolution supports a 10 4 yr reaction timeframe. Both of these rates are comparable to modern rates of dolomite dissolution (0.3 to 4.5 × 10 −4 mmol dolomite kg H2O −1 yr −1 ) calculated from measured reaction times in the Tertiary Floridan aquifer system in Florida and the Madison aquifer in the Mississippian Madison Limestone of the Northern Great Plains. Similarity of these rates to the estimated paleo-rates of dolomite dissolution supports a 10 4 yr reaction timeframe. The Holocene reaction time also can be compared to a series of reaction times calculated by assuming that the mass of dolomite missing from the Edwards was removed at rates observed in the Floridan and Madison aquifers. These reaction times (for complete removal of dolomite) range from 2700 to 58,500 yr and span the Pleistocene-Holocene boundary. Finally, an estimated dolomite reaction rate during dedolomitization of the Edwards aquifer based on surface area of exposed dolomite [mmol cm −2 s −1 (millimoles per square centimeter per second)] may be approximated from reaction times. This rate is directly a function of the mass of dolomite removed and the surface area exposed per pore volume passing through the rock. The surface area is available from the observed dolomite rhomb size in unaltered rock. The rate of pore fluid movement is obtained from the averaged annual discharge. Rates during formation of the Edwards aquifer calculated from all reaction times range from 10 −13 to 10 −14 mmol dolomite cm −2 s −1 . These rates are faster than rates (10 −18 mmol cm −2 s −1 ), measured in the pure laboratory system, CaMg(CO 3 ) 2 CO 2 H 2 O, but slower than rates determined in an alpine stream study (10 −10 to 10 −11 mmol cm −2 s −1 ) where cold glacial melt water flows over dolostone. Dolomite dissolution rates from both the Edwards and other aquifers support the concept that a major part of the Edwards aquifer could have formed within the Holocene.