Groundwater Recharge, Discharge, Runoff Conditions And Chemical Constituents Formation Mechanism In The Coastal Plain, Downstream Of Yangtze River

Fresh water is very precious resource in coastal areas. With the development of economy, the demand for fresh water is increasing. Coastal aquifers suffer from overexploitation which causes environmental geological problems. The coastal plain of downstream of Yangtze River has large amount of groundwater. Vertically, there are five aquifers, unconfined aquifer, the first, second, third and fourth confined aquifer. The third confined aquifer is the main production layer because of good water quality and quantity. However, many years of overexploitation make the water level fall rapidly and the regional depression cone formed. The water becomes salinization as well. In order to reasonable development of groundwater and control of deep fresh groundwater salinizaton, the recharge, discharge, runoff conditions, the salinity distribution and the hydrogeochemical processes of groundwater must be fully understood in the context of regional and local hydrogeological conditions.This paper is funded by “groundwater circulation and water chemical evolution of Jiangsu coastal area”. On the basis of detail analysis of geological and hydrogeological conditions, the data of water level, geochemistry and environmental isotopes are used to do this research. The recharge, discharge, runoff conditions of groundwater and the water chemical constituents formation mechanism of every aquifers are studied in this paper. Main conclusions are as follows.1. The unconfined aquifer is mainly recharged by precipitation, discharged by evapotranspiration. It flows slowly(flow velocity is only 0.2 mm/d) from the inland to the sea. The first and second aquifers are hydraulic connected. They are recharged by the lateral flow. The first confined aquifer can also be recharged by river water near the Yangtze River. The first and second confined aquifers are discharged by lateral flow except the northern area where they are discharged by artificial IV exploitation. They flow from the inland to the sea except the northern part where they flow to the mining center. Their flow velocity is 3 mm/d and 4 mm/d. The third and fourth artesian water has little hydraulic connection between each other. They are also recharged by lateral flow, and the third confined water can also be recharged by the second confined water in the Ru Dong and Qi Dong littoral area. The third and fourth confined aquifers are discharged by artificial exploitation which makes the water flow toward the depression cone. Their flow velocity is 12 mm/d and 8 mm/d.2. The tritium concentration of groundwater shows that, the unconfined water is recharged by sub-modern and modern precipitation. The first to fourth confined water is recharged prior to 1952. They are not recharged by modern precipitation. The first to fourth water’ 14 C age are, 9100~10400 a, 8400~15000 a, 12500~>30000 a, 16700~>30000 a. They are recharged by the last stage of late Pleistocene to early Holocene precipitation. The 14 C age of the third and fourth confined aquifers increase from the west to the east which means that the recharge area for them is to the west of the study area. Combined with the regional hydrogeological condition, the recharge area is located around the city of Yang Zhou and Zhen Jiang.3. The TDS distribution of unconfined aquifer and the first to fourth confined aquifers are similar, which increases gradually from inland to the sea. The unconfined water, the third and the fourth confined water is mainly fresh water. The first and second confined water mainly consists of saline water. As the TDS increases, the major anion of groundwater changes from bicarbonate to chloride and the major cation is sodium remaining the same. The vertical salinity distribution of aquifer matrix soluble salts shows that the study area has experienced many times of marine transgression and regression. The transgression scope forms a shape of wedge. There exists an area of desalinization in the near surface(about 10 meters thick).4. The chemical constituents formation mechanism in each layer has a certain difference. The unconfined groundwater chemistry is controlled by evapotranspiration, water rock interaction, mixture with recent and paleo seawater and human activity. The first and second confined water is with marine features and are significantly affected by paleo seawater. Apart from conservative mixing, base exchange, sulfate reduction and carbonate precipitation also ocurr. Fresh water chemistry in the third confined aquifer is mainly controlled by silicate weathering and base exchange. The brackish water in the third aquifer originates from mixing with the upper saline groundwater. The fourth confined fresh groundwater chemistry is controlled by the same hydrogeochemical processes for the third confined water. The salinity of brackish water in the fourth confined aquifer originates from basin salt accumulation.