Identifying Priority Areas for Riparian Rehabilitation to Minimise Nitrate Delivery to Streams

Surface water and groundwater systems are connected with the head gradient between the river and the nearby aquifer controlling the magnitude and direction of the exchange flux between the two systems. The direction of the flux dictates whether the river gains water from the nearby aquifer, or loses water to it. The exchange between groundwater and rivers is a key component influencing not only river discharge/recharge (from a quantitative perspective), but also affects water quality, geomorphic evolution, riparian zone character and composition, and ecosystem structure. A number of processes contribute to the exchange flux between surface and groundwater, most importantly they include: aquifer recharge (including diffuse recharge, recharge from irrigation return, and recharge from overbank flow), bank storage, groundwater extraction, and evapotranspiration. Riparian zones can provide a protective buffer between streams and adjacent land-based activities by removing nitrate from shallow groundwater flowing through them. The flow of shallow groundwater through the biologically active root zone of riparian vegetation, which is inherently rich in organic carbon, facilitates the denitrification process. Hydrological factors are an important influence on the effectiveness of riparian buffer zones in reducing pollutant loads delivered to streams. Nitrate-bearing water can interact with sediments in many forms. The classical conceptual model for nitrate attenuation during base flow conditions in gaining streams suggests that groundwater travels laterally and interacts with the riparian soil before discharging to the stream. In this work, conceptual models for surface water-groundwater interactions are presented along with analytical mathematical functions that describe nitrate removal in riparian zones. These concepts were incorporated within a GIS modelling framework to develop the Riparian Mapping Tool (RMT), which assesses the potential of riparian zones to reduce nitrate delivery to streams. The RMT was adopted to prioritise riparian rehabilitation in the Tully catchment (QLD, Australia) with high-priority areas defined as those having a high potential for riparian denitrification and nearby land uses that generate high nitrogen loads. 1.0 Introduction Aquatic organisms require nutrients for their metabolism, growth and reproduction, but when present in excess, nutrients are considered to be pollutants that can have adverse impacts on ecosystem health. Recent studies in several parts of Australia have shown nitrogen to be a key nutrient likely to trigger algal blooms and related problems in both coastal waters (e.g., Dennison and Abal, 1999; Murray and Parslow, 1999) and freshwater bodies (e.g., Mosisch et al., 1999, 2001). Sources of nitrogen associated with land use include fertilizer application, soil erosion, inputs from human and animal wastes, and in some cases, precipitation (e.g., from vehicle emissions in large cities) (Hunter et al., 2006).