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~ Q ~ ~ ".:") :) "< v -''-' The ion and TDS budgets for Lake Mead and Lake Powell revealed that the concentrations of certain ions were drastically altered by the impoundments (Figure 2). Lake Mead increased average sulfate concentrations by 63 mg/l during the 1951-60 period which was over 50 mg/l higher than that expected due to net water losses (Figure 2). There was less increase in sulfate during the 1970-79 period in Lake Mead, but concentrations were still 26 mg/l higher than ex- pected (Figure 2). Lake Powell elevated average sulfate con- centrations by 24 mg/l in the 1970-79 period which was about 10 mg/l higher than expected (Figure 2). . The increases in sulfate were offset to varying degrees by reductions in carbonate concentrations (Figure 2). In Lake Mead, carbonate was reduced by 29 mg/l below the ex- pected value during the 1951-60 period (Figure 2). Silica and calcium were also slightly lower than expected. These 'redljctions were not sufficient to offset increases in s1.l1-. fate, and TDS concentrations rose by 42 mg/l. [TDS concen-, trations expressed as the sum of constituents do not always agree with TDS measured as residue. Both are reported in Figure 2, but we only refer to sum of constituents.] In the 1970-79 period, average carbonate concentrations were only 9 mg/l lower than expected for Lake Mead (Figure 2). TDS con- centrations therefore increased by 57 mg/l which was 15 mg/l higher than the previous period and 12 mg/l higher than expected. Lake Powell reduced carbonate by 14 mg/l which offset nearly one-half the increase in sulfate (Figure 2). TDS concentrations increased by 34 mg/l and were only slightly higher than expected during the 1970-79 period. The concentrations of other ions were not altered appreciably by the impoundments (Figure 2). There was no measureable change in magnesium and potassium concentrations in Lake Powell during 1970-79 or in Lake Mead during 1951- 60. These ions only increased by 4 mg/l in Lake Mead during the 1970-79 period. Sodium and chloride were slightly higher than expected in Lake Powell during 1970-79 and in Lake Mead during the 1951-60 period. However, during the 1970-79 peri- od, sodium and chloride concentrations were lower than ex- pected for Lake Mead (Figure 2), indicating that these ions were being retained in the impoundment. This seemed unlikely because of the conservative nature of both ions. Rather, it appears that sodium and chloride loading to Lake Mead are being underestimated because of a sampling error that developed in Grand Canyon after flows were regulated by construction of Glen Canyon Dam in 1963. We explain this in greater detail in the discussion section of the paper. 7