Lacustrine groundwater discharge and its nutrient contribution in a large shallow lake of China

Shallow lakes are characterized by a larger sediment area and the stronger aquatic-terrestrial exchange of nutrients, therefore are generally more productive and vulnerable to be eutrophicated than deep lakes. Lake Taihu is the third largest freshwater lake in China, and is a typical shallow lake with the maximum depth of 3.3 m. It suffers from serious eutrophication and river inputs and sediment diffusion are thought to be the main sources of nutrients and have been primarily targeted in traditional aquatic remediation. The nutrients from groundwater have been ignored. the Quaternary aquifers are well-developed around the lake and highly enriched in nutrients. The concentration of dissolved inorganic nitrogen (DIN) in the shallow aquifer is averaged to be 811.4 mol L-1, much higher than those in river water and lake water of 204.5 mol L-1 and 65.1 mol L-1, respectively. Lacustrine groundwater discharge (LGD) could be a significant pathway for delivering nutrients to Lake Taihu. This study makes the leading perform of natural radioactive tracer radon (222Rn) to in a large shallow lake, combining with other hydro-geochemical analyses for the seasonal samples of different water endmembers around Lake Taihu, to quantify the LGD and LGD-borne nitrogen loads. The 222Rn distribution in lake indicates that LGD occurs not only along the lakeshore but also through the bottom of the shallow lake. The mass balance model of 222Rn leads to LGD estimates of 5.4±1.7 mm d-1 in dry season and 41.7±10.4 mm d-1 in wet season. And a three-endmember model based on water and Cl- budgets produces a relatively small LGD estimate of 1.6±1.0 mm d-1. LGD in Lake Taihu is a significant component of water budget and takes up 34.4-67.3% in water input. LGD-borne nutrients can reach 3-6 times of those from rivers. Groundwater can potentially contribute large amounts of nutrients, leading to the environmental and ecological problems in Lake Taihu. It suggests that LGD needs to be considered in future environmental protection and ecological restoration of Lake Taihu. The study further reviewed other similar shallow lakes and found that LGD rate tends to be positively related with the lake surface area, but data of more great lakes are needed to confirm this relation.