the world". Other introduced species, like the carp and mosquitofish, have become major pests. Environmental issues The large catchment area of the Waikato River is highly fertile farmland, so intensive agriculture is present. Due to the agricultural activity within the catchment significant agricultural pollution is leached into groundwater and contained in the runoff. The mismanagement of nitrogen fertilizer and effluent spreading practices in dairy farming is seen as the major causes of this pollution. Since 2000 Environment Waikato has joined with conservation minded farmers to bring about more efficient and scientific use of fertilizers. The removal of the native vegetation throughout the catchment to accommodate the increasing demand for farmland has contributed to the silting-up of the river with loose soils from eroded farmland, although most of the silting is due to the construction of the many hydrodams. In its pre-1930s wild state, the silt was flushed from the river every winter by flood surges. The remnants of these can be seen in the silt channels carved out of what is now St Andrews golf course, adjacent to the river in Hamilton. Arsenic enters the river at concentrations that reach 0.035 grams of arsenic per cubic metre in places, exceeding the WHO provisional guideline of 0.01 grams of arsenic per cubic metre and making the water unsuitable for drinking water unless treated. The majority of arsenic in the Waikato River comes from the Wairakei Geothermal Power Station. The amount of arsenic gradually declines as the river flows northwards and is at its lowest at the Waikato River Heads. Since 2002, around of water a day has been drawn from the river at Tuakau, treated and pumped along a pipe north to Auckland, where it is mixed with local water. This met 8% of Auckland's water needs in 2010/11. In December 2012 capacity was increased to , and in 2013 work started to increase it to . The treatment plant meets New Zealand's 2000 drinking water standards according to Water Care NZ. This is equal to or better than the A standard for Auckland's other water supplies. Slightly-modified human wastes are pumped into the river downriver of several towns. Hamilton city has one of the most modern water treatment systems in the world following a 2003 report by GHB water consultants. In 2007 $22 million was spent upgrading the existing intake station south of the city at Riverlea. This will meet