THE CARBONATE HYDROGEOCHEMISTRY OF THE KRKA RIVER, CROATIA

The Krka River is incised into a broad plateau underlain by folded carbonate rocks along the Dalmatian coast of Croatia, the type region of Karst topography. The Krka River drains a surface watershed of ~2,610 km2, and receives flow contributions from a network of interconnected subsurface watersheds of unknown configuration.

The Krka River is characterized by calcareous-tufa deposits that are model examples of fluvial barrage systems, where tufa accumulates in structures arrayed perpendicular to the flow of the river (Ford and Pedley 1996). To determine the mechanisms responsible for tufa precipitation, we investigated the carbonate chemistry, temperature, pH, and topography of the river and accessible parts of its watershed. We determined the carbonate content of water in the Krka and some of its tributaries by in situ titration every ~2.0 km along the 72-km length of the Krka. Values of pH ranged from 7.4 to 8.4; the concentration of total carbonate ranged from 300 to 500 ppm. We collected 32 samples of modern tufa from sites close to the titration stations, and analyzed those samples by X-Ray Diffraction. Within limits of analytical precision, all samples were identical in mineralogy, with calcite the predominant mineral. Aragonite, magnesium calcite, magnesite, stenonite, and quartz were identified at much smaller percentages.

In tufa-precipitating river systems with well characterized hydrologic inputs, such as Havasu Creek, AZ, or the Mopan River, Belize, pH and carbonate content follow a systematic trend of rising pH and decreasing carbonate content with distance from the source. In the Krka system, that simple pattern is not realized; sequential subsurface inputs apparently alter the hydrochemistry and, ultimately, the process of tufa precipitation. Within the Krka system, we identified three short segments of the river through which systematic rise of pH and decline in total carbonate content document distance downstream from a discrete subsurface source. Those segments are separated by reaches in which variation of those chemical parameters follows no clear pattern, suggesting that subsurface inputs to the Krka vary both in space and time.