The Management of Non-point Source Pollution in Rural Catchments

"1995".

Project Number: Management of Non-point Source Pollution in Rural and semirural Catchments; MDFRC Project M/03/5109 & SB/1/24; NRMS Project M202; LWRRDC Project MDR6.

MDFRC item.

1 of 2 reports associated with project see (Diffuse nutrient loads entering the Murray River from the Long Plain Creek via the Carroll’s Lane Drain, Howlong, NSW).

The progressive eutrophication of water draining agricultural catchments has caused increasing concern in recent years. The source of nutrient pollution in such catchments is difficult to assess, measure and control, as it is widespread and diffuse. Land holders and communities who are responsible for managing productive rural and urban catchments face the dilemma of maximising biological productivity on the land and maintaining it at a high level, while minimising the risk of the undesirable side effects of high biological productivity in associated aquatic ecosystems. Resolution of this dilemma requires that ways be found to retain nutrients within the productive catchments and to recycle them on a sustainable basis. The use of wetlands to intercept diffuse nutrient runoff has been proposed recently as a strategy for the control of diffuse pollution, particularly those nutrients important in eutrophication. The aim of this study is to provide data on the impact of three wetlands on nutrient loads fl owing from small rural catchments. This study was considered necessary as there are few rigorous data sets available for Australian conditions which assess the performance of wetlands for diffuse pollution control. Three wetlands in north east Victoria were studied between 1992 and 1995. Two of the wetlands were located along small drainage lines and may be more correctly termed•”vegetated drainage lines”. One of these was a natural wetland while the other was constructed although this structure quickly silted up and for all intents and purposes acted as a natural system after the first year. Results from these wetlands indicate that they function as nutrient sinks (ie. more nutrient enters the system than leaves) most of the time, particularly for phosphorus. However. they are capable of discharging net amounts of stored nitrogen and phosphorus both in storm events and between events in background flows . A nutrient budget for both linear wetlands in a below average rainfall year February 1994 to January 1995 indicated that they intercepted 11% and 23% of the nitrogen load and 17% and 38% of the phosphorus load for Crookes and Humphreys wetlands respectively. The third wetland, a two hectare Phragmites swamp with a small catchment. was expected to achieve high retention of nitrogen and phosphorus. Surface water flows and nutrient loads calculated for storm and background flow events indicated that this natural wetland was a net exporter of nutrient. This was primarily due to the wetland being a ground water discharge area . A joint study with the former Rural Water Commission (Hydrotechnology) was undertaken to monitor and model ground water loads. Many natural wetlands on the floodplain are impacted by ground water and an appreciation of this aspect of nutrient budgets is important. Preliminary analysis and modelling indicate that the nutrients in the ground water provide most of the nutrient load leaving this wetland in the surface outflow. However, initial estimates of ground water and surface water loads indicate that ground water loads alone cannot account for all the additional nutrient load leaving the system. The additional nutrient may originate from a fire which burnt the wetland. although increased loads leaving the wetland immediately after the fire were not detected. The other possibility is that the ground water mobilises stored nutrient from the wetland under certain conditions which still have to be identified. This study has not identified the reasons for retention o r release at this stage. A more detailed examination of the data is in progress. it is likely that there are complex interrelationships between the magnitude, timing, duration and chemical nature of the nutrient load entering the wetland and morphological, chemical. biological and hydrological processes within the wetlands which will change in time and space. Small wetlands in rural catchments will most likely have the greatest impact on the settling and filtration of particulate material. The study has indicated that there is potential for numerous small wetlands to have a cumulative impact on diffuse nutrient loads flowing from catchments, even in large storm events. This may be facilitated by encouraging farming communities to maintain wetlands and to fence off small drainage lines where possible. Most rural landscapes consist of a mosaic of ecological systems that will affect the water quality of run-off in a variety of ways. It is likely that any attempt to control diffuse pollution from rural catchments will require the use of several systems, such as wetlands, in a balanced flexible strategy that may have to be tailor-made for each catchment. This in turn requires a good understanding of the different ecosystems that could form part of the composite strategy as it is unlikely that any one system will behave consistently and reliably with respect to its nutrient relationships. Thus. an ecologically sustainable solution will entail a holistic approach requiring sensitive management of a number of compatible ecosystems within the landscape. The results of this project should contribute to this. Management Recommendations. On the basis of the information obtained in this study. and the collective experience and judgement of those who worked on the project. the following management recommendations are made. 1. On farm wetlands should be preserved. rehabilitated or created. 2. Small wetlands in series within sub-catchments which have a cumulative impact on nutrient loads are likely to be more efficient and cost effective than larger structures at the lower end of catchments due to the nature of nutrient loads generated in storm events. 3. Opportunities for wetland establishment should be sought as part of land and water management planning on a catchment by catchment basis and at the individual property scale. 4. Farm planning principles should include water quality management using wetlands, fenced waterways and drainage lines. 5. Consideration should be given to retarding run-off flows where practicable. This may mean a reassessment of current farm drainage practices. 6. Where possible, small drainage lines should be fenced off and the growth of emergent aquatic and riparian vegetation preserved or encouraged. 7. If planting is needed, the use of emergent aquatic vegetation which is indigenous, and which will provide a uniform barrier to runoff is recommended. 8. As appropriate, simple earth works should be instituted to maximise delivery of runoff water to wetlands and to increase their retention times. 9 . Ground water discharge areas are a nutrient source in the landscape and present particular problems. As well as adding to the nutrient load they may disrupt the retention capacities of associated wetlands. At the present level of knowledge attempts should be made to incorporate discharge into wetlands/retarding basins below the discharge area. 10. Special management requirements such as nutrient cycling through controlled grazing, periodic extraction and harvesting etc. will be necessary as small wetlands will saturate over time. 11. Management issues such as weed and pest control will need to be addressed. 12. The use of wetlands for diffuse pollution control will not be a total solution. but will be a valuable tool as part of a larger integrated strategy. Research Recommendations: The following hypotheses have been formulated during this project and it is recommended that further research be conducted in the following areas. 1. The phosphorus load retained by a wetland in a storm event is released in a more bioavailable form in low flows after the event. (Test if the wetland converts particulate bound phosphorus to more bioavailable forms). 2. The phosphorus load retained by a wet land in a storm event is subsequently flushed from the wetland during storm events in the following 6 months. (Test if the wetland acts primarily as a temporary store). 3. The filtering and sedimentation of particulate material is the main mechanism of nutrient (N and P) interception in small wetlands with a limited hydraulic retention time. (Measure the major mechanisms involved in nutrient interception in these wetlands). 4. Phosphorus retention efficiency in small wetlands is governed by the size distribution of particulate phosphorus in incoming flows and the hydrology (flow rate and duration) of the event which will affect settling rates and residence times. (Measure the range of particle sizes in the material intercepted by the wetland during different storm events). 5. The macrophytes which provide the most impediment to flow (shape. distribution, uniformity and density of culm distribution) will intercept the most nutrient. (Test which macrophytes are most effective in impeding flow) . 6. High flows in large storm events are not impeded by macrophyte vegetation and consequently do not lose nutrient. (Test whether during high flows water passes over the top of Ule vegetation and bypasses nutrient interception mechanisms.) 7. The redox potential of the substrate is the significant parameter that influences chemical change to more/less "stable" forms of major nutrients important in release/ retention mechanisms during low flows only. (Test the significance of the redox conditions present in the substrate during background flow conditions.) 8. Recycling of nutrients from small wetlands (grazing, harvesting, physical extraction of accumulated silt and humic material) is possible without severe disruption to the wetland and its interception capacity. (Test the effects of