Geochemistry and anthropogenic inputs of metals in a tropical lake in Venezuela

Lake Valencia is a tropical lowland lake in north-central Venezuela. Lake bottom sediments were collected from 25 locations in April, 1988. At 2 locations water pH, conductivity, dissolved O2 and temperature were measured at successive depths. After drying, 5 sediment samples were sieved into 5 mechanical fractions. Each was extracted with 1 M HNO3 and analysed for AI, Ca, Cd, Co, Cr, Cu, Fe, K, Mg, Mn, Na, Ni, Pb and Zn. The <63 μm fraction of all samples was similarly analysed. Water temperatures declined with depth (1°C/25 m) from approximately 26°C. Conductivity varied little with depth but dissolved O2 contents indicated anoxic conditions < 25 m. Water pH (8.8–9.4) was consistent with high dissolved carbonates. There was little consistent relationship between grain size and sediment metal contents. Approximate baseline metal contents were calculated from 21 of 25 samples. The average composition of the sediments corresponded to an ideal mixture of shales/carbonate rocks as 0.5–0.7/0.5-0.3. Five samples from alluvial fans near the mouths of rivers traversing urban-industrial zones had compositions different from the other sediments. Generally, Ph, Zn, Ni, Cd and Cu were anthropogenically enriched by factors of 2–16. The major sources of pollutants were identified as domestic and industrial activities affecting the rivers that traverse the lakeside cities of Maracay and Valencia. Sodium, Mg, Ca, Mn and CO3− showed natural enrichment arising from carbonate precipitation following a physical mixing of 2 sedimentary components (biogenic carbonate and terrigenous material). Low geochemical mobility of metallic elements in neutral-basic and reducing waters, a processes of bury and mixture of sediments and precipitation of carbonate result in only 10% of the lake area being affected by contamination. Isoline plots of elements in the bottom sediments supported a hypothesis that material mixing, physical transport and carbonate precipitation are the main controls of spatial distribution patterns.