LAKE CAVIAHUE, ARGENTINA: NON STEADY STATE DYNAMICS IN A VOLCANICALLY ACIDIFIED LAKE

Many lakes are not in steady state with respect to recent pollutant inputs. The master equation for pollutant dynamics of P is dP/dt = IN – OUT, where P is the total amount of the pollutant. In many lakes, the output terms are a function of the pollutant concentration, and the OUT terms can be phrased as a series of k*P terms which can be phrased explicitly if occurring sequential or simultaneously (k=removal rate). For a constant influx with conservative element flushing, we obtain p(t) = p(ss)*(1-e^t/RT) + p(init) e^t/RT, where p(t) is the time dependent pollutant concentration, p(ss) is the steady state concentration and p(init) the natural or initial concentration, and RT is the residence time (and k=1/RT). The equation can be used to forecast pollutant concentrations over time or constrain input fluxes from time series of analytical data. I apply these principles to Lake Caviahue, a large (0.5km³) glacial lake acidified by hyperacidic effluents from an active volcano (Copahue), a natural analog for pollutant dynamics. The acid volcanic springs have pH ~ 0-1.5 and the local river that enters the lake has pH ~1-2.5, whereas the lake water and outflow have pH ~ 2.2-3. A long lake-water compositional record provides insights into the variations in the volcanic inputs, mineral saturation in the lake waters, as well as variations in the composition of the input. Discharges were measured once a year, providing element fluxes into the lake over the 17 year period. Large variations were observed in K and Al concentrations, which relate to episodic alunite crystallization inside the volcano, with an associated decrease in permeability and strongly reduced effluent fluxes. The lake waters were at Schertmannite saturation when the pH value approached 3, and the outflow had its river bed covered with Schwertmannite. The mineral precipitation led to strong reductions in dissolved P, As, and V, which are adsorbed/incorporated into the mineral as their respective oxyanions.