Lake Rotorua sediment survey

Water quality in Lake Rotorua has improved over recent years, with reductions of nitrogen, phosphorus and chloropyll a concentrations, and increased water clarity. These changes coincide with alum dosing of the Utuhina and Puarenga inflows, commenced in 2007 and 2010 respectively. The aim of inflow alum dosing is to reduce external phosphorus load by binding (inactivation of) phosphorus flowing into the lake, however, it has been hypothesised that dosing these streams has had subsidiary beneficial effects on the water quality in the lake itself. Namely, the hypothesised effects relate to adsorption of P in the water column and the settling of alum flocs onto bottom sediments, which in turn could reduce sediment release of phosphorus into the overlying water column (internal loading). This report was commissioned by Bay of Plenty Regional Council to investigate the possibility of stream-dosed alum improving water quality in Lake Rotorua. The objective of this study was to assess how much of the dosed alum, if any, could have been deposited onto the bottom sediments of Lake Rotorua where it could reduce internal phosphorus loading. This objective was achieved by two approaches: (i) a field survey of Lake Rotorua bottom sediments in order to observe any changes in sediment aluminium and phosphorus concentrations following the alum dosing of Utuhina and Puarenga streams and (ii) an aluminium mass balance for the inflows and lake, to explore the potential amount of alum settling out of the water column and onto the bottom sediment, taking into account the potential phosphorus uptake rate of alum. In comparison with sediment samples collected in 2006, sediment Al concentrations in 2012 were lower in the main basin of the lake. Phosphorus concentrations in the sediments were reduced at four sites located near to the discharges of the Utuhina and Puarenga Streams, but sites further from the stream (i.e. deep basin and close to the Ohau Channel outlet) showed increased concentrations of phosphorus in the sediment. The aluminium mass balance for Lake Rotorua provided insights into the potential amount of alum reaching the lake and sediments after some proportion of the aluminium had bound phosphorus in the preceding ‘reactor’, including the inflows. There was a low probability that a large proportion of the dosed Al would have reached the sediments in Lake Rotorua. Only in the years 2010 and 2011, when alum dose rate to the streams was comparably high, some of the dosed alum was likely to have reached the lake sediments. There was a high probability that a large fraction of the Al reached the lake water column with the potential to reduce in-lake water column P concentrations. Scenarios in the aluminium mass balance were developed to ascertain the conditions needed to achieve a given target for percentage uptake of lake water column P, with respect to the parameters for the fraction of TP available for uptake by alum and the effective Al: P ratio. Only when simulating a 90% reduction of water column P concentrations was the range of possible parameter values for achieving such a reduction relatively constrained. In simulated scenarios where a substantial reduction of internal P loading was the target, parameter values in both the inflows and water column were considerably constrained, indicating that phosphorus uptake by aluminium was likely to be considerable in the inflow and water column. Even with constraint of the parameter values in the inflows and water column, it appeared unlikely that an amount of alum could reach the sediment sufficient to significantly reduce internal P loading. Changes in sediment P concentrations observed in this study are suggested to have occurred through the synergistic effects of changes in external nutrient loading, sediment focusing and changes in duration and severity of anoxic events in the hypolimnion of the lake driven by unusual climatic events during that time which may have reduced water column stability. The alum mass balance results suggest that although it is possible that some alum reached the bottom sediments in Lake Rotorua, any effect on internal loading was likely to be minor, even in the years 2010 and 2011 when the alum dose rates were comparably high. The conditions under which a substantial reduction of internal P loading would have been possible (with respect to the fraction of the P pool available for uptake and the effective Al:P ratio), are considered highly optimistic (i.e. Al:P ratio ~ 3.5) when compared to literature values of Al:P ratios in other alum-dosed eutrophic lakes. This study suggests that the amount of Al dosed to the streams is unlikely to explain completely the recent improvement of water quality in Lake Rotorua. Any claim otherwise, should be viewed with caution and can be regarded as optimistic