A combined field and modeling study of groundwater flow in a tidal marsh

Abstract

Abstract. Bald mud beaches were found among the mangrove marshes in Dongzhaigang National Nature Reserve, Hainan, China. To investigate the possible reasons for this phenomenon, the intertidal zones of a mangrove transect and a bald beach transect with similar topography and tidal actions were selected for comparison study. Along both transects, observed water table variations were significant in the high and low intertidal zones and negligible in the middle intertidal zones. Despite the same tidal actions and above-mentioned similarities, observed groundwater salinity was significantly smaller along the mangrove transect (average 23.0 ppt) than along the bald beach transect (average 28.5 ppt). These observations invite one hypothesis: the hydraulic structure of tidal marsh and freshwater availability may be the main hydrogeological factors critical to mangrove development. Two-dimensional numerical simulations corroborated the speculation and gave results in line with the observed water table. The two transects investigated were found to have a mud-sand two-layered structure: a surface zone of low-permeability mud and an underlying highpermeability zone that outcrops at the high and low tide lines. The freshwater recharge from inland is considerable along the mangrove transect but negligible along the bald beach transect. The high-permeability zone may provide opportunity for the plants in the mangrove marsh to uptake freshwater and oxygen through their roots extending downward into the high-permeability zone, which may help limit the buildup of salt in the root zone caused by evapotranspiration and enhance salt removal, which may further increase the production of marsh grasses and influence their spatial distribution. The bald beach is most probably due to the lack of enough freshwater for generating a brackish beach soil condition essential to mangrove growth. It is also indicated that seawater infiltrated the high-permeability zone through its outcrop near the high intertidal zone, and discharged from the tidal river bank in the vicinity of the low tide line. These processes thereby formed a tide-induced seawater-groundwater circulation, which likely provided considerable contribution to the total submarine groundwater discharge (SGD). Finally, implications and uncertainties behind this study were summarized for future examinations.

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

“…The tidal contribution to SGD was estimated to be ∼20 m 3 d −1 m −1 . Xia and Li (2012) quantified the tide-induced SGD in a tidal marsh using the numerical code MARUN, which was ∼6.0 m 3 d −1 m −1 with no inland freshwater recharge. The seepage face development was also ignored.…”

Quantifying tidal contribution to submarine groundwater discharges: A review

“…(Figure 6(a) and (c)) the tidal contribution to SGD can be estimated to be ~20 m 2 /d. In 2012, Xia and Li [42] used the numerical model MARUN reproduced the field observed watertable in two transects (one is mangrove transect and the other is tidal flat) in mangrove marshes in Dongzhaigang National Nature Reserve, Hainan, China. They identified that both transects have a mud-sand two-layered structure: a surface zone of low-permeability mud and an underlying high permeability zone that outcrops at the high and low tide lines.…”

“…During the circulation of seawater, seawater mixes with the terrestrially-derived groundwater and chemicals, leading to various biogeochemical interactions in the pore water in rocks and soils. Therefore, SGD is an important source of nutrients, contaminants, chemicals and trace elements to the coastal waters [1,24,[38][39][40] and has considerable impact on the quality and the ecosystem of the receiving seawater [41][42][43][44].…”

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

“…A series of experiments conducted in gravel beaches of Prince William Sound, Alaska, indicated that the twolayered structure of a high-permeability surface layer underlain by a low-permeability layer had significant impacts on the persistence of spilled oil ( Guo et al, 2010;Li and Boufadel, 2010; . 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.…”

“…Seepage meters were also used to estimated SGD at local area, such as manual seepage instrument developed by Cable et al ( 1997 ) , heat pulse seepage meter developed by Taniguchi and Fukuo (1993) and ultrasonic flow meter developed by Paulsen et al (2001) . Numerical simulations usually focused on limited coastal transect, such as Guo et al (2010) , , Xia and Li (2012) , Xia et al (2010) .…”