Waituna catchment water quality review

Abstract

The Waituna catchment in coastal Southland flows into Waituna Lagoon, a brackish lagoon and wetland complex that falls within the category of Intermittently Closed & Open Lagoon or Lake (ICOLL) coastal water bodies. In recent years, the lagoon has undergone rapid deterioration in water quality and ecological health. The lagoon is already eutrophic, and is tending towards a sudden loss of original benthic macrophyte coverage (dominated by Ruppia sp.), which could lead to the lagoon having an undesirable turbid, murky water dominated by algal slime. One of the causative factors attributed to the eutrophication process is the contribution of freshwater inflows of nitrogen, phosphorus and suspended sediment from farming activities in the Waituna Creek catchment. These nutrients are transported to Waituna Lagoon by surface and sub-surface flow paths, including the catchment creek network. Land use in the Waituna catchment is predominantly high productivity improved pasture that has been established by a long-term process of land clearance and drainage of formerly scrub covered land and wetlands. Relevant to the effects of this land use change process on water quality, is that high productivity pasture generates higher nutrient and sediment losses than the original native vegetation. The Waituna Creek network drains entirely into Waituna Lagoon, which is blocked from outflow to the sea for the majority of the time by a barrier spit. Consequently, a large proportion of these contaminants can become trapped in lagoon water column, sediments and biomass. The Waituna catchment comprises the following sub-catchments: • Waituna Creek • Moffat Creek • Carran Creek • Craws Creek • Lagoon margins catchment In addition to the surface water network water also flows to the lagoon via the shallow gravel aquifer and to a lesser extent the deeper Gore Lignite Measures multi-layer groundwater system. We found limited evidence from existing lignite mining and groundwater studies to suggest that there is significant interchange between the lignite measures groundwater system and surface waters that would influence the lagoon’s nutrient loading. However, the shallow gravel aquifer in the upper and mid catchment zones show evidence of dynamic transfer of nitrate nitrogen from oxidised brown (Waikiwi) soil series with high leaching potential, through the shallow aquifer. This nitrate-enriched groundwater augments Waituna Creek flow immediately downstream of the Mokotua settlement, particularly from the early winter period. We have examined the Mokotua Infiltration Zone (MIZ) concept which was proposed in the most recent groundwater technical report on the Waituna catchment (Rissmann, et al., 2012). The occurrence of nitrate-enriched base-flow in Waituna Creek has been confirmed by a subsequent Surface Water Quality Study of surface water composition. However, our evaluation of the mechanisms for nitrate nitrogen infiltrating to groundwater, seeping into creek water, and the timing of these processes, differ from those of the original MIZ concept. There is evidence that areas of the Waikiwi soil type in the upper Waituna Creek catchment, upstream of Mokotua settlement, are associated with the infiltration of significant loads of nitrogen to the shallow aquifer. Nitrogen appears to accumulate in the soil profile and shallow aquifer over the drier, warmer months when potential evapotranspiration is high, and soil drainage is low. We predict that up to 90% of the annual nitrate nitrogen mass load may be flushed into Waituna Creek during late autumn to early winter. The soils are vulnerable to rainfall-driven leaching events at this time because of lowering evapotranspiration rates and soil nitrogen uptake. We estimate that area-based nitrogen loadings of 42 to 64 kgN/ha mobilised from the Waikiwi soils upstream of Mokotua settlement during this time, and are discharged to Waituna Creek over a 1 to 2 month period. Once the stored nitrate nitrogen in soil and shallow groundwater is exhausted by winter flushing, subsequent winter and spring soil leaching events contain low nitrogen loads. A contrasting set of geochemical processes applies elsewhere in the Waituna catchment, associated with large areas of podzols, gley, and organic soils. The soils form anoxic conditions in shallow groundwater where oxygen reduction occurs, including the denitrification of Dissolved Inorganic Nitrogen such as nitrate. Anoxic, or reduced, groundwater geochemistry also leads to a decline in the phosphorus buffering capacity of soils due to the release of oxidised iron (Fe³⁺) and a subsequent reduction in phosphorus adsorption. Accordingly, nitrate nitrogen concentrations are very low in areas of reducing soils, while dissolved phosphorus may rise to concentrations not usually observed in oxic groundwater. We have also assessed the catchment boundaries assigned by Environment Southland on the basis of land surface gradient. It appears that the actual contributing areas of different sub catchments are difficult to clearly define because of the following tendencies: • Manipulation of drain base slopes can produce an artificial drainage pattern different to the drainage patterns implied by land surface slope • Catchment flow divides are quite unclear where they cross peat wetlands, such the Waitana catchment adjoining the Awarua Wetland complex • Shallow groundwater in the north of the upper Waituna Creek has a flow gradient that may flow beneath the surface water divide with the Waihopai catchment In the latter case, the presence of significant areas of Waikiwi soils in the Waihopai catchment may enable additional nitrate nitrogen to contribute to Waituna Creek base-flow via the shallow aquifer. A similar, but potentially less significant, inflation in Carran Creek headwaters’ nitrogen load may also occur.