The impact of seepage influx on cation content of a Central Amazonian floodplain lake

We give the first report on seepage dynamics in an Amazonian floodplain lake, by directly quantifying seepage influx, and qualifying its impact on the hydrochemical seasonality of Lake Camaleão, an island channel lake, which is subject to the 10 m floodpulse of the lower Solimões/Amazon River. The lake shows a distinct seasonal pattern in cation concentrations, with a rise and maximum during its low-water (isolation) phase. To quantitatively and qualitatively clarify the significance of seepage influx, a series of hydrological and hydrochemical variables was recorded in 1998. The observed seasonal pattern in the content of solute cations was similar to results reported for 1995 [G.E. Weber, Causes of hydrochemical seasonality of major cations in Lago Camaleão, a Central Amazonian floodplain lake, Ver. Int. Verein. Limnol. 26 (2) (1997) 408-411.]; when first qualitative evidence for the largely seepage driven process of cation loading was found. Concentration of the monitored Ca, Na, Mg and K, in Lake Camaleão and Solimões were similar during high water. Solute cation content in the groundwater around Lake Camaleão was up to 20 times higher than in lake and river water. With the beginning of falling water, cation concentrations started to rise slightly in Lake Camaleão. Starting with isolation from Solimões River the content of solute cations in the lake rose by approximately 150% within 14 days, then the concentrations had reached their maximum values. This falls in with 14 days of cation rich seepage influx into lake Camaleão. Mean seepage influx rate during this period was 1.45 mm/h, which equals a total influx into Lake Camaleão of 9.7 m³/min. After 14 days seepage stopped to enter the lake, lake water began to seep into the sediment, (seepage flux inversion), and the rise in cation concentrations in Lake Camaleão came to an end. Based on the water budget and measured cation concentrations of the contributing water fluxes we predicted cation concentrations at the end of the 14 days period of seepage influx into the isolated Camaleão. Observed and expected cation concentrations are fairly close. The coincidence of seepage influx and the solute cation increase in the isolated Camaleão, observed twice in 1998 and 1995, gives us strong evidence that cation loading by seepage is the main cause for the lake's characteristic peak of cation concentration during falling water. Our field measurements correspond to values calculated by [K. Furch, Evaluation of groundwater input as major source of solutes in an Amazon floodplain lake during the low-water period, In: Verh. Int. Verein. Limnol. 27, Stuttgart, Germany, 1999.] considering changes in ion concentrations which also point to groundwater as major ion source for Camaleão Lake at low-water period.