Water quality in the Oldfield and Munglinup Rivers: an identikit picture

January 2001

Andy Chapman has been enthusiastically collecting water quality information in the Oldfield estuary catchment since May 1998. The measurements are made at a number of sites on the Oldfield and Munglinup Rivers and in various smaller tributaries throughout the catchment. It is not until all seasons and a good variety of flow conditions have been sampled that a reasonable picture of the catchment water quality can be pieced together. For example the January 2000 flooding was an unusually intense event that will have effected the summer and autumn situation this year. The aim is to gather data over two years, not a long time by monitoring standards, but it will highlight certain catchment characteristics and assist planning for future assessments.

The 1998/99 results will be presented in a lot more detail at a seminar to be held in the area in July this year. The water quality information may confirm observations made by community members over the years and for some it may provide a fresh and maybe unexpected insight into what is happening in their waterways.

A few preliminary observations of salinity values suggest that the Oldfield and the Munglinup Rivers both become less salty as they approach the inlet. The values in the tributaries are high from the middle to upper part of the agricultural area, but lower nearer the coast. This probably reflects the rapid change in rainfall with distance from the coast and is a feature of other south coast rivers. It may also suggest something about the relative groundwater contributions in different areas. Although, overall, the water in the lower Oldfield River is less salty, the reaches can be quite fresh at times. When the flow into pools ceases the pool will steadily become saltier as the water evaporates, and so higher salinities in pools can be part of the seasonal cycle.

Total phosphorus values have been encouraging and generally under the desired upper limit of 0.1 milligram/litre, however the tributaries have from 30% to 50% of their values above this limit. The preferred upper limit for total nitrogen is one milligram/litre. Over half of the values are above this limit, in the main channel of the rivers but particularly in the tributaries. A few values are quite high, up to five milligrams/litre. The sampling site at the Springdale Road crossing nevertheless had all values of total phosphorus and total nitrogen within acceptable limits and this may reflect the ability of the bush corridor along the river to strip nutrients from the water as they percolate down from the agricultural areas.

The estuary monitoring part of the program has confirmed that the Oldfield estuary is in excellent condition with little evidence of problem algae. In this respect, along with Broke Inlet near Walpole, it is a valuable environmental asset.

The generous vegetated corridors along the rivers provide sediment trapping and nutrient filtering to water flowing off the land. The progress of their condition over the next few decades will provide an important benchmark for other south coast catchments and their associated estuaries.