Using Rn-222 to Study Human-Regulated River Water-Sediment Input Event in the Estuary

The implementation of the water sediment regulation scheme (WSRS) is a typical example of artificially controlling land-source input. During WSRS, the water discharge of the Yellow River will increase significantly, and so will the input of terrigenous materials. In this study, we used a natural geochemical tracer Rn-222 to quantify terrestrial inputs under the influence of the 2014 WSRS in the Yellow River Estuary. The results indicated that during WSRS the concentration of Rn-222 in the estuary increased by about four times than in the period before WSRS. The high-level Rn-222 plume disappeared quickly after WSRS, indicating that Rn-222 has a very short 'memory effect' in the estuary. Based on the investigation conducted from 2015 to 2016, the concentration of Rn-222 tended to be stable in the lower reaches of the Yellow River. During WSRS, the concentrations of Rn-222 in the river water increased sharply at about 3-5 times greater than in the non-WSRS period. Based on the Rn-222 mass balance model, the fluxes of Rn-222 caused by submarine groundwater discharge (SGD) were estimated to be (3.5 +/- 1.7) x 10(3), (11 +/- 3.9) x 10(3), and (5.2 +/- 1.9) x 10(3) dpm m(-2) d(-1) in the periods before, during, and after WSRS, respectively. This finding indicated that SGD was the major source of Rn-222 in the Yellow River Estuary, which can be significantly increased during WSRS. Furthermore, the SGD-associated nutrient fluxes were estimated to be 9.8 x 10(3), 2.5 x 10(2), and 1.1 x 10(4) mu mol m(-2) d(-1) for dissolved inorganic nitrogen, phosphorus, and silicon, respectively, during WSRS or about 2-40 times greater than during the non-WSRS period.