Magela Creek system, northern Australia. I. 1982-83 wet-season water quality

Abstract

The particularly late wet season in 1982-83 provided a unique opportunity to study the dry-to-wet season transition in Magela Creek, a tropical creek and wetlands system in northern Australia. The first water to flow down and across the floodplain was characteristically quite acidic (pH c. 4-5), with a high conductivity (c. 750 µS cm^-1) and high sulfate concentration (c. 200 mg I^-1). The source of the sulfate, acidity and dissolved salts is groundwater brought to the surface (mainly in depressions on the floodplain, e.g. pools and channels) by rising watertables, and then flushed from these by the slowly advancing flood waters. The end of the dry season is a time of particular stress for the biota living in these billabongs, and many fish kills have been noted at around the time this 'first flush' water enters. In most years, this poor-quality water would be rapidly flushed out, in a few days at the most, from the billabongs by subsequent flood- flows down Magela Creek. However, in 1983, the next flood event did not occur until almost 1 month after the first flush had occurred.

During the main wet season, the water transported by Magela Creek was slightly acidic (mean pH 5,2), with a very low conductivity (c. 5-17 µS cm^-1) and low concentrations of suspended solids (c. 4-59 mg I^-1), major ions and trace metals (iron, manganese, copper, lead, zinc, uranium). There was a general decrease in the mean concentration of each determin and with each succeeding flood event. This was most noticeable in the case of suspended solids and conductivity, and was attributed to an 'exhaustion effect' where greater amounts of suspended solids and soluble salts (in the upper soil layers) were available in the early part of the wet season. The composition of the creek water was very similar to that of rainwater falling in the catchment, with two major exceptions. First, the rainwater was considerably more acidic (pH 3.6-4.9), due mainly to the presence of weak organic acids such as formic and acetic acids. This acidic rainwater was partially neutralized by interaction with catchment soils. Second, the concentrations of all nutrients (total phosphorus, nitrate-nitrogen and ammonia-nitrogen) were considerably higher in the rainwater. There was a significant removal (over 90%) of phosphorus, ammonia and nitrate from the rainwater, probably due to uptake by the ground vegetation known to proliferate in the catchment during the wet season. Factor analysis indicated that dilution of the base flow, presumably by surface runoff of rainwater, was the dominant component during both the rising and falling stages of each flood event.

Radionuclide activity in composite water samples taken over the five flood events revealed that the total activity of both ²²⁶Ra and ²¹⁰Pb was low, with greater than 58% of both radionuclides being in filterable forms.