Occurrence and Quality of Ground Water in Crystalline Rocks of the Llano Area, Texas

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Abstract
Median yield of wells in crystalline rocks of the Llano area is 14 gpm. Near the Highland Lakes the median yield is somewhat higher (17 gpm) owing to continuous recharge from the lakes. Domestic wells near the lakes are not as deep (median depth = 66 feet) as in the rest of the area (median depth = 76 feet). The major ground‐water supplies occur in granite grus and fractured granite.The yield per saturated foot of aquifer decreases with depth, owing to (1) the decreasing degree of weathering and hence of porosity and permeability with depth, and (2) the closing of fractures with depth. The decrease with depth of incremental yield is more marked in the Llano region than in the Sierra Nevada and in the eastern United States. Optimum depth of wells in the area is considered to be 75 to 100 feet. It is commonly more economical from the standpoint of well yield to drill several shallow wells rather than one deep well.Quality of ground water is generally good, but erratic, with median concentrations (in mg/l) of Si, 49; SO4, 61; Cl, 83; NO3, 21; Fe, 0.07; and total hardness (as CaCO3), 302. Schist generally yields water with higher total hardness and sulfate concentrations than other rock types. The water samples with high nitrate concentrations typically are from wells near septic systems or livestock pens. Even in ground water away from pollution sources the dissolved solids content tends to be higher than in water within crystalline rocks of other geographic areas.

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