Recent research in Nature affirms the role of targeted wetland restoration in reducing nitrate loads to ground and surface waters. Coming in at a price-tag of about US$3.3 billion annually across the USA, they say that a spatially targeted increase in US wetland area by 10 per cent (5.1 million hectares) would provide an estimated 54 per cent decrease in nitrogen loading in nitrate-affected watersheds such as the Mississippi River Basin.
The paper cited is available here.