Salinity

Salinity at the Loveday Disposal Basin

Despite the influx of freshwater during the experimental flooding, the salinity of the sediments at Loveday remained extremely high. Salinity was highest for the samples collected from the surface of the basin, indicative of evaporation, and decreased with depth. Electrical conductivity for the surfacesediment samples ranged from ~70 mS/cm for the North Basin, ~50 mS/cm for the South 97

Basin and down to 17 mS/cm at the Museum site. The electrical conductivity decreased dramatically to values of <5 mS/cm immediately below the surface at all sites, and remained at these values down to the water table.

The salt chemistry of the surface sediments, determined from 1:5 sediment to water extracts, is dominated by Na+ _{and Cl}-_{. Chloride concentrations ranged from ~70000 mg/kgfor the North}

Basin, 40000 mg/kg for the South Basin and 30000 mg/kg for the Museum site. Sodium

concentrations ranged from ~40000 mg/kg for the North Basin, 23000 mg/kgfor the South Basin, and 17000 mg/kgfor the Museum site. The concentrations of sodium and chloride decrease with depth, as do those of other major cations and anions. The decrease in the salt concentration with depth was not as pronounced at the Museum site as it was elsewhere in the Loveday Disposal Basin.

After chloride and sodium, the next most abundant ion in the sediment extracts was sulfate. The sulfate concentration of the sediments was highest for Loveday North, where it was ~30000 mg/kg, with lower concentrations of ~10000 mg/kgin the South Basin and~9000 mg/kg for the Museum site. The high sulfate concentration in the sediments contributes to the accumulation of sulfide in the sediments through microbial metabolism.

The experimental reflooding had minimal impact on salinity. The Loveday Disposal Basin remained highly saline after the flooding, and any benefit from the influx of freshwater would have been short-term. During the experimental reflooding, there was density stratification as the fresh input water overlaid the more saline water which was already present in the basin. Salinity levels fell to <15 mS/cm during the reflooding with salinity levels returning to the levels seen previously, >60 mS/cm (Lamontagne et al., 2008). To manage salinity these wetting-drying cycles would need to be combined with flushing flows to remove the salt from the system.

Salinity at Mussel Lagoon

The sediments at Mussel Lagoon were exposed at the time of sampling in 2008. The surface sediments were found to be saline, with an electrical conductivity of ~5 mS/cm. Analysis of these sediments showed the salinity is dominated by sulfate, with a concentration of 14000 mg/kg, which is higher than that found at Loveday South or the Museum site. The high concentration of sulfate in these sediments results from the formation and subsequent oxidation of sulfidic material in this wetland. This finding was unexpected, as Mussel Lagoon was not thought to be affected by sulfidic material. However, the data suggested that, prior to the drying régime imposed on the site as a water-saving measure, a significant quantity of sulfidic material was present at this site. The high sulfate concentrations at Mussel Lagoon are confined to the surface sediments, so the sulfides that formed in this wetland formed very recently, rather than over a relatively long period

of time as at Loveday. Now that this wetland has such elevated concentrations of sulfur within its sediments, the system may degrade further through enhanced sulfur cycling.

Actual and Potential Acidity

Given the quantity of reduced sulfur contained in the sediments and the imposition of a prolonged drying phase, there was a risk of acidification of the sediments contained in all wetlands. An initial part of this research project was to determine the effects of the experimental reflooding on the Loveday Disposal Basin in terms of acidity generation and the distribution of reduced sulfur in the sediments. Mussel Lagoon was incorporated to allow for a comparison between fresh and saline wetlands.

6.2.1.

Actual Acidity

The acidity at Loveday is confined to the very recent sediments within 40 cm of the surface, representing deposition since the Basin was commissioned as a salt disposal basin in the 1970s (Gell, 2007). There is a horizon at approximately 20 cm which typically has a pH below 5. This was encountered in the South Basin (the 400 sites) and at the 202 site in the North Basin, nearest the outer margin of the wetland. These sites would have been exposed to the atmosphere for much longer than the other sites sampled at Loveday.

When sampled in 2008, Mussel Lagoon was found to contain acid sulfate materials at a depth of 0-5 cm. The concentration of sulfur in these sediments is a consequence of reduction of dissolved sulfate, which fixed sulfur into the wetland sediments as insoluble metal sulfides. This sulfur has since oxidised, and lowered the pH significantly, with some strongly acidic sites exhibiting pH values below 4 in this wetland. However, when the wetland was sampled in 2010, while it was inundated, the pH of the water in the wetland was 6.35, indicating the system had recovered and much of the acidity had been neutralised.

The formation of acid sulfate materials at Mussel Lagoon may be related to the differences in hydrologic connectivity between this site and the Loveday Disposal Basin and the Berri Evaporation Basin. Alkalinity is generated during the microbial formation of sulfides. Mussel Lagoon is connected to the Loveday Disposal Basin through surface networks, so water has been released into the Loveday Disposal Basin during the experimental reflooding, as well as by previous unauthorised release through vandalism, has exported alkalinity from the Mussel Lagoon system.

The steep pH gradients found in the sediments of both Mussel Lagoon and the Loveday Disposal Basin are very significant. The changes can be from pH 8 to pH 4 across a vertical distance 5 cm, representing a 10 000 fold change in acidity. This steep pH gradient is confined to the upper ~30 cm of sediments, with the pH increasing to >7 and being sustained down to the water table. For 99

the Loveday Disposal Basin, this gradient gives an indication that the buffering capacity of the surface salt crust, 0-5 cm, is high, whereas the buffering capacity of the sulfidic material below the surface, 5-30 cm, has been exhausted and continued oxidation may lead to the formation of sulfuric material.