CARBONATED MINERAL SPRINGS OF SARATOGA SPRINGS, NEW YORK: MIXING END MEMBERS AND WATER-ROCK INTERACTIONS

Fifteen slightly acidic, cold, reducing, carbonated mineral springs occur in Saratoga Springs and Ballston Spa, NY, near the trace of the Saratoga Springs-McGregor fault. Though these waters have been studied for ~200 years, they are still poorly understood with regard to the source of water end members, the composition of the most mineralized end member, and water-rock interactions before and after mixing between the end members. The spring waters are dominated by the anions Cl and HCO₃^-, and by the cations Na, Ca, and Mg. They range from 0.15% TDS (Total Dissolved Solids) to 1.84% TDS, whereas local surface and shallow well waters have ~0.03% TDS. The most mineralized spring water available at the surface has a calculated P_CO2 of 5 or 6 bars prior to degassing. Spring waters are, in part, mixtures between a fresh water and a highly mineralized water end member. The straight mixing line, as defined for example by the conservative components Cl and Br, is distorted most notably for Ca, Mg, and HCO₃^- because of partial reequilibration of the mixed waters with aquifer rock. Extensive geochemical modeling was done using the U.S.G.S. PHREEQEC program. 1) Models show that the fresh water end member was probably saturated with calcite but undersaturated with dolomite. 2) The highly mineralized water end member is undersaturated with dolomite at all reasonable P_CO2 values by a small amount (may be a software artifact). 3) All end member mixtures should be undersaturated with both calcite and dolomite, with maximum undersaturation occurring in a mix having ~7% of the most mineralized water end member. For individual springs dolomite saturation ranges from 0 to almost 100% (with respect to a straight mixing line), with most springs being 10 to 40% of saturation. This is unexpected because the Beekmantown aquifer rocks have abundant calcite and dolomite, and because water can equilibrate rapidly with carbonate rocks (days or weeks). It seems unlikely that the fresh water end member comes from the Adirondack foothills northeast of the springs, traveling through kilometers of carbonate rock. The fresh water end member is therefore probably local. Mixing apparently takes place soon before rise of the water to the surface, limiting reaction with aquifer rock.