Estimation of seawater–groundwater exchange rate: case study in a tidal flat with a large-scale seepage face (Laizhou Bay, China)

Abstract

The exchange rate between seawater and groundwater in a tidal flat was investigated at Laizhou Bay, China, where there are large-scale seepage faces with horizontal extension of several hundred meters developed during low tides. Taking into account the effects of seepage face and density, a simple and efficient method for estimating seawater-groundwater exchange rate is proposed, based on field measurements of groundwater hydraulic head, temperature and salinity. First, the exchange rate at each well was obtained using the generalized Darcy's law, then the results were interpolated and integrated along the whole transect. The total submarine groundwater discharge (SGD) and inflow were estimated to be 8.8 and 15.3 m 3 d −1 m −1 , respectively. The spatial distributions of SGD and inflow were different from those of sandy or gravel beaches possibly owing to the low-permeability sediment (silty sand with mud), very gentle slope, and the large-scale seepage faces. A freshwater discharge tube was identified near the lowtide line, as evidenced by significant increase in outflow and low salinity of groundwater observed there. The SGD from the seepage faces accounted for ∼21 % of the total SGD. The outflow rate that occurred from the seepage faces, and the ratio of the outflow from the seepage faces to the total outflow, decreased seaward significantly and monotonically.

Seawater-groundwater Exchange in a Silty Tidal Flat in the South Coast of Laizhou Bay, China

“…The impacts of density and groundwater pumping for salt production on SGD and inflow were discussed quantitatively. The difference between the results of this study and those of Ma et al (2015) were analysed. Comparisons were made between the single-well and pair-wells methods.…”

“…More details can be seen in Cox, Culkin, and Riley (1967) ; Ma et al (2015) ;and, Wooster, Lee, and Dietrich (1969) . Then, the observed pressures of air and groundwater, and elevations of each LTC-Diver, were transformed into freshwater-equivalent hydraulic head (Figure 4).…”

“…When the beach surface is submerged by seawater, h surface equals the freshwater-equivalent hydraulic head of the tidal level; otherwise, it equals the beach surface elevation during the beach surface exposure. More details about the single-well method are shown in Ma et al (2015) .…”

“…Xia and Li ( 2012 ) made a comparative study between a bald beach and a mangrove beach with similar topography and tidal actions in the intertidal zones of Dongzhaigang in Haikou City of Hainan Province, indicating that the inland freshwater recharge and the mud-sand two-layered structure played key roles in the development of mangrove and their spatial distribution. Ma et al ( 2015 ) developed a simple method ( " single-well method" ) for estimating seawater-groundwater exchange rate in a silty beach, taking into account the effects of seepage face and density and based on field measurements of groundwater hydraulic head, temperature and salinity. The exchange rate can be calculated by the generalized Darcy's law, the observed tidal level and the hydraulic head in the observation well, Their case study in a silty beach at Haimiao of Laizhou Bay indicated that the SGD and inflow were 8． 8 and 15． 3 m 3 d -1 m -1 , respectively.…”

“…In this paper, " pair-wells method" is used to improve the single-well method used by Ma et al (2015) to investigate the seawater-groundwater exchange rate in a typical cross-shore siltybeach transect in the intertidal zone of Qingxiang Town ( Liutuan Town) , Laizhou Bay. Fourteen pair-wells were used to monitor the pore water pressure, salinity and temperature at two different depths simultaneously along the entire transect.…”