The influence of colloidal material on aluminium speciation and estimated acid neutralising capacity (ANC)

Abstract

Cationic Al species (Al i) cause toxic effects towards fish in acidified water. As several factors can influence the Al i-speciation, acid neutralising capacity (ANC) has been applied as an alternative indicator for acidification and effects towards fish. However, the critical range of ANC for biological stress has been shown to be dependent upon the concentration of organic material (DOC). Using in situ size fractionation techniques the influence of particulate and colloidal DOC (0–400 μM) on Al i-speciation and ANC-values was investigated in Norwegian streams. During high flow events with high river transport of DOC the Al i concentration increased by a factor of 2 due to retention of colloidal Al species (>10 kDa), probably organic, in the chromatography column. Ultrafiltration (⩽10 kDa) of water prior to chromatography reduced the influence of non-toxic organic Al-species, on Al i-speciation. The charge balanced ANC-values (ANC cb) were also significantly lower (on average 34 μeq L −1) in ultrafiltered water compared to unfiltered water samples, as base cations were associated with colloidal DOC (>10 kDa to 0.45 μm) and organic acids were not incorporated in the ANC cb estimate. Thus, ANC cb will underestimate the acidification effects towards fish in organic waters increasing with concentration of colloidal DOC present. Alkalinity based ANC-values (ANC alk), which include a fraction of organic acids, were similar in unfiltered and ultrafiltered waters, but higher than ANC cb-values of ultrafiltered samples. Thus, ANC alk-values also underestimate the acidification effects on fish in organic waters. Based on ultrafiltered samples, however, ANC cb-values was negatively correlated with Al i independent of the DOC present and thus a more correct indicator for toxic Al-species.