Are groundwater inputs driving de-oxygenation, acidification, and eutrophication in northern NSW rivers and estuaries?

A project undertaken at the Centre for Coastal Biogeochemistry, Southern Cross University and supervised by Isaac Santos

A number of East Australian estuaries are severely and recurrently impacted by de-oxygenation, acidification, and eutrophication. These issues may cause total fish mortality and have a major regional economic impact. Increasing nutrient inputs from agriculture and urban activities leads to harmful algal blooms and a reduction in aquatic biodiversity.


Pollutant concentrations are often orders of magnitude higher in groundwaters than in surface waters, suggesting that groundwater may be a chronic but unquantified source of acidity, anoxia, and nutrients to northern NSW estuaries. Estimating how groundwater contributes to these issues can be extremely expensive and laborious if done by traditional means (numerical modelling or hydrogeologic investigations). As a result, groundwater has been put in the “too hard basket” for decades and no quantitative information exists on groundwater discharge into most Australian estuaries.


This project has demonstrated the use of an efficient method to assess groundwater inputs into surface waters based on automatic radon (a natural groundwater tracer) measurements. We have addressed three major scientific questions:

  1. What are the areas where groundwater discharge can be qualitatively important?
  2. What are the groundwater discharge rates in these areas?
  3. What are the groundwater-derived fluxes of acidity, anoxia, and nutrients?

Our project revealed a strong link between groundwater discharge and surface water quality in the Richmond River Estuary.

The following 4 scientific papers have been have been published:

Santos, I.R., de Weys, J., Tait, D.; Eyre, B.D. 2013. The contribution of groundwater discharge to nutrient exports from a coastal catchment: Post flood seepage increases estuarine N:P ratios. Estuaries and Coasts, in press.

Santos, I.R., de Weys, J., Eyre, B.D., 2011. Groundwater or Floodwater? Assessing the Pathways of Metal Exports from a Coastal Acid Sulfate Soil Catchment. Environmental Science & Technology 45, 9641-9648.

Santos, I.R.; Eyre, B.D. 2011. Radon tracing of groundwater discharge into an Australian estuary surrounded by coastal acid sulphate soils. Journal of Hydrology, 396: 246-257.

de Weys, J.; Santos, I.R.; Eyre, B.D. 2011. Linking groundwater discharge to severe estuarine acidification during a flood in a modified wetland. Environmental Science & Technology, 45 (8), 3310-3316.

The following Honours theses have been supported at least in part by this project:

de Weys, J. 2010. The contribution of groundwater discharge to estuarine acidification during a flood (Tuckean Swamp, northern NSW). Honours Thesis. School of Environmental Science and Management, SCU, Lismore, NSW, 67 p.

Gleeson, J. Submarine groundwater discharge and its biogeochemical implications in disturbed and natural estuarine systems. Honours Thesis. School of Environmental Science and Management, SCU, Lismore, NSW, 54 p.

Makings, U. Using radon to determine whether groundwater is a major source of nutrients to a subtropical tidal river and estuary. Honours Thesis. School of Environmental Science and Management, SCU, Lismore, NSW, 54 p.

Atkins, M. 2012. Groundwater discharge as a significant source of carbon dioxide to a coastal floodplain creek. Honours Thesis. School of Environmental Science and Management, SCU, Lismore, NSW, 80 p.

The thesis by de Weys has been turned into a scientific journal article. The other 3 theses above are available under request and will be prepared for publication in a scientific journal.

 

Figure 1. The map shows the distribution of radon (a groundwater tracer) and pH in the Tuckean Swamp, a major coastal acid sulphate soil in northern NSW. The survey revealed two contrasting areas: (1) natural creeks (i.e., Stibbards Creek) where groundwater discharge does not cause surface water acidification; and (2) artificial drains surrounded by acid sulphate soils where groundwater discharge is clearly driving severe surface water acidification.  Our automated radon measurement system can be used to detect areas of enhanced groundwater inputs.

 

Figure 2. A student sampling groundwater in a coastal acid sulphate soil catchment. Notice the stained vegetation as a result of iron precipitation under oxic conditions.