Nutrients
Pollution of surface and groundwater resources by nutrients is of growing concern worldwide as high nutrient concentrations can have harmful effects on both ecosystem and human health. Understanding river-aquifer connectivity is an important aspect of nutrient management. In addition to natural sources such as soil and organic matter, agricultural and urban activities are also contributing nutrient contamination to surface water and groundwater systems. Nutrients of greatest concern are nitrogen (N) and phosphorus (P).
Impacts of nutrients on surface waters and groundwaters
Eutrophication is a condition found in aquatic ecosystems whereby high nutrient concentrations, especially nitrogen (N) and phosphorus (P), stimulate excessive production of aquatic plants, including algae. Both nutrients occur in numerous chemical and physical forms and originate from a variety of diffuse and point sources. Although eutrophication is a natural process in the aging of lakes and some estuaries, human activities such as agricultural runoff, urban runoff, leaking septic systems, sewage discharges and eroded stream banks can increase the flow of nutrients and organic substances into aquatic systems. Algal blooms on the surface of the water prevent the penetration of sunlight essential for underwater plant life while decay of algae depletes dissolved oxygen levels in the water, essential to most aquatic organisms. This disrupts the health and diversity of fish, plant and animal populations in the aquatic environment. Blue-green algae blooms can also contain compounds toxic to humans and stock.
Human health can also be affected by ingesting waters high in nitrate (NO3-). Bacteria in the digestive tract convert (reduce) nitrate to nitrite which can be absorbed into the bloodstream and oxidise haemoglobin. The methaemoglobin that forms is unable to function in the transport of oxygen to the tissues and in high enough concentrations can lead to the disease Methaemoglobinaemia. Infants under the age of 3 months are particularly vulnerable, and the disease can be fatal in this age group.
Nutrients in Australian waters
In Australia, the relative importance of different sources of nitrogen and phosphorus varies between catchments. In general, phosphorus tends to be bound to sediment whereas nitrogen is predominantly found in the dissolved state. According to the Australian Agriculture Assessment 2001, the dominant sources of phosphorus are hill slope erosion, gully and bank erosion, as dissolved form in runoff as well as in urban point discharges. Dissolved nitrogen in runoff comprises a greater proportion of the total load than dissolved phosphorus. Total nitrogen loads are mainly derived from hill slope erosion and dissolved nitrogen in runoff. Approximately 19,000 tonnes of total phosphorus and 141,000 tonnes of total nitrogen per year are exported down rivers to the coast from areas of intensive agriculture.
Nitrate in Australian groundwaters can be sourced from point or diffuse sources similar to those contributing to surface waters.
Significance of groundwater-surface water interactions in nutrient management
In landscapes where groundwater and surface waters interact, the occurrence of high levels of dissolved nutrients can have implications for water quality. For example, the discharge of groundwater high in nitrogen into lakes, wetlands, streams and marine environments can cause eutrophication of the receiving surface water body and cause algal blooms to develop. In Australia, groundwater has been found to be a significant source of nitrogen to freshwater (Lamontagne 2002), estuarine (Linderfelt and Turner 2001) and coastal environments (Smith et al. 2003).
Modelling of nutrient budgets has traditionally focussed on the major pathways for nutrients transported through surface water systems. However, a missing link in these budgets is the contribution of dissolved nutrients from a groundwater source, in the case of a gaining river system, or losses, in the case of a stream that loses water to the subsurface. For catchments where groundwater is a significance source of nutrients to surface waters, there is likely to be an underestimation of nutrient inputs, which due to lag times may not be realised for many years (Lamontagne et al, 2003).
Case Studies
Herbert River Catchment, North QLD
Relevant Links
National Land and Water Resources Audit Australian Agriculture Assessment 2001
National Land and Water Resources Audit Australian Natural Resources Atlas
UK Environment Agency Attenuation of nitrate in the subsurface environment
References
Lamontagne S, Herczeg AL, Dighton JC, Pritchard JL, Jiwan JS, Ullman WJ, 2003. Groundwater-surface water interactions between streams and alluvial aquifers: Results from the Wollombi Brook (NSW) study (Part II - Biogeochemical processes), CSIRO Technical Report 42/03.
Lamontagne S, 2002. Groundwater delivery rate of nitrate and predicted change in nitrate concentration in Blue Lake, South Australia. Marine and Freshwater Research, 53: 1129-1142.
Lawrence CR, 1983. Nitrate-rich groundwaters of Australia. Australian Water Resources Council Technical Paper No. 79.
Linderfelt WR, Turner JV, 2001. Interaction between shallow groundwater, saline surface water and nutrient discharge in a seasonal estuary: The Swan-Canning system. Hydrological processes, 15: 2631-2653.
xSmith AJ, Turner JV, Herne DE, Hick WP, 2003. Quantifying submarine dishcharge and nutrient discharge into Cockburn Sound, Western Australia. Centre for Groundwater Studies Report No. 104, 185p.
Weier K, 1999. The quality of groundwater beneath Australian sugarcane fields. Australian Sugarcane.