We performed dissolved trace metal investigations in Double Crossing Creek, a tributary of Hearnes Lake estuary and part of the Solitary Islands Marine Park (SIMP), to assess the potential influence of horticultural activity and rain events on water quality.
Time series measurements of dissolved trace metals was undertaken from 27 January to 3 April 2018 over multiple hydrological regimes. After a 109 mm rain event on 25 February, Hearnes Lake began to drain to the ocean. After the estuary drained, streamflow past our sample site was exclusively from the upstream catchment.
The concentrations and export rates of dissolved trace metals in Double Crossing Creek increase after rainfall events of various magnitudes. Rain events of 30 mm were sufficient to increase dissolved contaminant concentrations above the Australia and New Zealand Environment and Conservation Council (ANZECC) water quality guideline (WQG) values for both fresh and marine water.
Generally, export rates of dissolved contaminants were greatest when rainfall exceeded 50 mm. The elements mercury (Hg), copper (Cu), and zinc (Zn) exceeded either the ANZECC freshwater or marine WQG for more than 20 % of sampling events, in both wet and dry conditions. During and within 24 hours after rain events, Hg and Zn exceeded ANZECC WQG by more than 10 fold during our sampling. Flushing of agricultural soils containing these trace metals is believed to be the source of dissolved Hg, Cu, and Zn to Double Crossing Creek.
Estimated export rates of dissolved trace metals were high compared to examples from the literature on a per area basis.
Comparisons of trace metal export from Double Crossing Creek to subsequent sediment burial rates of trace metals in Hearnes Lake revealed the estuary may be a source of dissolved Hg, Cd, and Mn to a sanctuary zone of the SIMP at times when the estuary is hydrologically connected to the ocean. At times when Hearnes Lake is closed, the estuary is believed to retain upstream inputs.
No data on the ecological effect of trace metal export was collected. We suggest analyses of the chemical speciation and biological accumulation of trace metals (especially Hg) is undertaken to better understand the ecological implications of trace metal export from this horticultural catchment to the SIMP.
To prevent dissolved contaminant exposure in areas downstream of intensive horticulture we recommend actions to reduce use of products which may contain Hg, Zn, and Cu and to minimize and capture runoff during rain events.