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<br />~ <br />Q <br />~ <br />N <br /> <br />It appears that significant changes have occurred in <br />the rates of dissolution and precipitation in Lake Mead <br />since early impoundment. Rates of gypsum dissolution de- <br />creased from an average of 123 mg/l during 1935-48, to 75 <br />mg/l during 1951-60 and 37 mg/l during 1970-79 (Table II). <br />Average rates of halite dissolution decreased from 19 mg/l <br />during 1935-48 to 3 mg/l during 1951-60. It was not possible <br />to compute rates for 1970-79. The changes observed in the <br />rates of dissolution in Lake Mead seem to confirm predic- <br />tions made by the USDI [15J that rates would decrease as the, <br />salt outcroppings dissolved or became silted over. <br />The changes in sulfate concentrations in Lake Mead may , <br />also have been influenced by the activity of sulfate reduc- <br />ing bacteria. These bacteria convert sulfate ion to hydrogen <br />sulfide under anaerobic conditions. The hydrogen sulfide <br />often combines with iron to form insoluble iron sulfide pre- <br />-<Jipi tates that are retained in the sediments [21 J. Howard ., <br />[10] noted that substantial populations of sulfate reducing. <br />bactE'ria were present in Lake Mead sediments. Rates of sul- <br />fate reduction have never been measured directly, but sul- <br />fate diffusion coefficients were determined for sediments in <br />Las Vegas Bay and Bonelli Bay [22J. It is possible to esti- <br />mate rates of sulfate reduction from sulfate diffusion coef- <br />ficients if we assume estimates for Las Vegas Bay are repre- <br />sentative of the Lower Basin of Lake Mead and those for <br />Bonelli Bay are representative of the Upper Basin. These <br />calculations indicate that sulfate reduction would decrease <br />sulfate concentrations in the outflow from Lake Mead by 8 <br />mg/l'.Yr. This functions to offset some of the increase in <br />sulfate concentrations caused by dissolution of gypsum. <br />Rates of calcite precipitation estimated by Howard [10J <br />indicated that 47 mg/l were precipitated annually in Lake <br />Mead during 1935-48 (Table II). Those measured by Prentki et <br />al. [16J were considerably lower during this period. Rates . <br />of calcite precipitation in Lake Mead decreased from an <br />average of 48 mg/l during the 1951-60 period to 15 mg/l <br />during the 1970-79 period (Table II) which does agree with <br />changes meas,ured in Lake Mead sediments and those expected <br />as a result of decreased phytoplankton productivity L16J. <br />The reductions in phosphorus loading that occurred after <br />Lake Powell was formed [23J caused productivity in the Upper <br />Basin of Lake Mead to decrease from an average of 4612 mg <br />C/m2'day to 503 mg C/m2'day [16,24J. Increased phosphorus <br />loading from sewage effluent discharges into Las Vegas Bay <br />increased productivity in the Lower Basin from 937 mg <br />C/m2.day to 1582 mg C/m2'day [24J. However, this was not <br />sufficient to offset the decreases that occurred in the <br />Upper Basin, and reservoir-wide productivity in Lake Mead <br />decreased by 78% after Lake Powell was formed in 1963. <br />Reservoir-wide calcite precipitation decreased from an <br /> <br />".-, <br /> <br />L~_, <br /> <br />'~ <br />(_~1 <br /> <br />., <br />" <br /> <br />".~ <br /> <br />10 <br />