Metal and phosphorus in bottom sediment of a small-sized reservoir in a rural mountainous catchment (NE Portugal): accumulation and geochemical mobility

Small-sized reservoirs have less capacity for the retention of sediment but are widely distributed in regulated basins. Therefore, small reservoirs collectively impart an important anthropogenic signature to the global sediment-flux, increasing the mean sediment retention when compared with estimates of mean sediment retention of large reservoirs.

A geochemical study of sediment-associated metal and phosphorus from a small-sized riverine reservoir, located in a mountainous rural region (Vila Real in NE Portugal), was conducted to evaluate the effectiveness of the reservoir as traps for these elements. The contents of metals and P were determined, as well as their spatial distribution pattern and their potential availability by using a four-step sequential extraction procedure for metals and the Chang and Jackson fractionation for P.

The metal contents in sediments were in the ranges of (µg/g): Cr (22-122); Cu (31-83); Ni (5-71); Pb (49-160); Zn (207-334). All the geochemical phases studied were important in the retention of the metals; within the most labile fractions, the reducible fraction was the most significant. The studied elements can be classed by potential relative mobility: Zn > Pb > Cu > Cr, Ni. The partition of elements contents through the geochemical phases and the balance between contents associated with the most mobile fractions and with the residual fraction suggest an important contribution from lithology to the total contents of Cr and Ni, and a significant contribution of anthropogenic activities to the contents of Cu, Pb, and Zn in the sediments from the reservoir. The analysis of the results on the geochemical partitioning of metals revealed to be important when the Sediment Quality Guidelines (SQGs) are considered. Phosphorus showed contents ranging between 1518-2454 µg/g; most samples revealed the predominance of the Fe-P fraction.

In general, the sediments of the reservoir showed maximum values of contents of metals above the Threshold Effect Level (TEL, µg/g: Cr-37.3; Cu-35.7; Ni-18; Pb-35; Zn-123). Chromium, Ni, Pb and Zn showed total contents exceeding the values of Probable Effect Level (PEL, µg/g: Cr-90; Cu-197; Ni-36; Pb-91.3; Zn-315). Chromium and Ni showed higher values than the reference ones, but these can be considered relatively unavailable since they are associated with the residual phase. The potentially available and/or total amounts of metals and P in sediments were relatively high, indicating that the quality of bottom sediments accumulated in this small-sized reservoir should be considered in management policies.