Improving water quality
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Overview
Several factors have increased the supply of sediment, nutrients and other contaminants to streams in many agricultural and urban catchments. These include: intensive land uses such as cropping, increased erosion as a result of clearing riverbanks; leaching and eroding of nutrients applied in fertiliser from the soil; uncontrolled stock access and the continuation of paddock or crop development up to the stream bank. These practices have resulted in bare soil and higher nutrient loads being delivered to streams and rivers.
Sediment and nutrients
Sediment and nutrient delivery are linked closely because much of the nutrient delivered to streams is attached to sediment particles, particularly clay particles; the main nutrients of concern in stream water are phosphorus and nitrogen. Other important contaminants include salt, pesticides, animal wastes and disease organisms. The main sources of these pollutants, which reduce water quality for all downstream users, are the hillslopes adjacent to streams, the channels themselves, and the baseflow that moves into the stream from adjacent groundwater.
Sediment, nutrients and other contaminants are transported from hillslopes to streams by way of runoff. Riparian land and its vegetation protect streams from the influx of such pollutants and, in this way, make a vital contribution to water quality. The protection mechanism is based on lower gradients, dense vegetation cover, and moist, organic-rich riparian soils.
Erosion
All stream channels erode. Stream erosion is a natural and essential process but in recent times has been accelerated by human impacts, often to unacceptable levels. Riparian and in-channel vegetation can protect the bed and banks from the three main mechanisms of erosion (sub-aerial, fluvial scour, and mass failure), but it is unrealistic to expect revegetation to eliminate all erosion. Streambank erosion is a dominant source of sediment and nutrients in many river systems, and sediment loads in Australian streams have generally increased by 10 to 15 times in intensively used river basins when compared with pre-European loads.
Other influences
Many streams are linked directly to the surrounding groundwater by permeable deposits of sands and gravels. Water can move between the two in either direction, depending on relative hydraulic gradients. Water moving into streams from groundwater as baseflow, either above or below the water level (or via the hyporheic zone of saturated sediments lying below and alongside the water column) can transport dissolved and colloidal materials such as salt, nutrients and pesticides. Riparian vegetation can influence this flow in some situations by taking up groundwater and nutrients and altering hydraulic pressures.
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30 August 2010 6:14 pm
