Land use intensification on the Canterbury Plains: What are the implications for water quality? Ken Taylor Environment Canterbury

“The vast majority of studies on cause effect relationships…conclude that agriculture, as typically practiced in New Zealand, has a detrimental effect on water quality. These studies have shown that high levels of sediments, nutrients, and bacteria are generally found in waterways in agricultural areas, and water quality guidelines are often compromised, particularly where farming activity is intensive” NIWA report April 2002

Land use intensification contents Potential effects Indicators of change Groundwater quality –Nitrate contamination - state & trends Surface water quality –Nitrate loads to spring-fed streams –Phosphorus –Microbial contamination –Sedimentation –Habitat loss –Loss of flow Managing nutrient leaching to groundwater – ECan approach

Land use intensification - potential effects Groundwater –Leaching of nutrients and micro-organisms - impacts on drinking water quality and “down- stream” effects

Land use intensification - potential effects Surface waters –Nutrient loads to spring-fed streams Nitrate toxicity Excessive plant growth Coastal lagoons Marine waters –Microbial contamination Recreational water quality Drinking water supply –Sedimentation –Habitat change Natural character Wetland loss Loss of shading –Loss of flow Habitat loss Water quality effects

Land use intensification – what’s happening out there?

Indicators of land use intensification… Tonnes of urea applied to Canterbury farmland

Indicators of land use intensification… 1995

Indicators of land use intensification… 2005

Canterbury Groundwater 75% of the region in some form of agricultural production Agriculture is the largest user of gw (84%). est. 350,000 ha is irrigated Untreated groundwater principal source of drinking water for communities, dwellings, & businesses (13%)

Schematic cross-section of the Canterbury Plains

Groundwater vulnerability

Canterbury Groundwater Deep unconfined aquifers & confined aquifers – water quality generally high Shallow unconfined aquifers & spring fed streams – water quality affected by human activities

What is the problem with nitrate? In groundwater –“universal” drinking water standard 11.3 mg NO 3 -N/L In surface water –Range of adverse effects Nitrate toxicity – ANZECC guidelines Excessive plant growth Coastal lagoons – e.g., Opihi Coastal waters

Coastal issues Increasing N loads to coastal waters  shift to ocean outfalls - community sewerage schemes e.g. Christchurch City, Waimakariri District, Timaru District - industrial discharges  rivers, coastal streams, springs – e.g. Ashburton Dist. - 2,500 tonnes N exported / yr under base flows, or 830 kg/km 2 – cf. Chesapeake Bay – 130 million tonnes N per yr, or 740 kg/km 2

What about phosphorus? Not a problem for groundwater Tends to move with sediment in surface water Most Canterbury streams P-limited What happens when P gets into surface water?

Sedimentation Is a consequence of the way land is managed –Intermittent inputs –Stock in or adjacent to streams –Drain-clearing Is a particular issue for streams that do not get flushed