Tidal Effects on Ground Water Discharge Through a Sandy Marine Beach

Abstract

Tidal fluctuations along the salt water boundary of a sandy beach affect the magnitude, location, timing, and salinity of both subaerial and submarine ground water discharge. Detailed studies of shoreline discharge from an unconfined aquifer at two sites in an embayment on the Cape Cod, Massachusetts, coastline provide insight into the highly dynamic spatial and temporal nature of discharge along sandy beaches affected by the tide. The constantly moving tidal boundary over a sloping beach results in a shoreline‐perpendicular discharge zone of 10 to 20 m, with ∼35% to 55% of the discharge being submarine discharge. The distribution of fresh ground water through a beach face varies greatly, depending primarily on the tidal cycle and range, the heterogeneous characteristics of the beach sediments, and the beach geometry. The estimated relative volume of discharge varies temporally with tidal fluctuations, with the greatest discharge occurring during early to mid ebbing tide and location of greatest estimated discharge moving seaward during ebbing tide. This is determined using net hydraulic head calculations in monitoring wells set in a shoreline‐perpendicular transect in the beach. The salinity of discharge varies temporally from near fresh water values of 1 part per thousand (ppt) to near coastal salt water values of 30 ppt, being saltiest at the start of discharge as the tide ebbs and freshest during a low tide period of ∼2 h. Of the discharge volume, ∼65% to 85% is estimated to be from salt water that infiltrates during high tide episodes. This study highlights the complexity of the dynamic coastal ground water discharge phenomenon and provides insight into the hydraulic mechanisms involved. While there is a general pattern to sandy beach discharge, comparison of results from beaches studied at Cape Cod indicates that the temporal and spatial details of the discharge is very site‐specific.

Submerged Groundwater Discharges as Source of Fecal Material in Protected Karstic Coastal Areas

“…SGD flux variation highly depends on tide level [34] that could generate depth changes (±60 cm). The vertical movement produced by the tide changes the flux patterns of the SGD in a tide cycle [35]. Then, in high tide conditions, the marine water column blocks or diminishes SGD.…”

“…Nortes season leads to a higher flow of the spring than in dry season; thus suspended materials could more easily transport from a wide distance. However, this pattern could also be explained by variations in the sedimentation process due to flow rate in the spring [46], for example, at low flow rate, as during flood tide particles settle down at a preferred distance of 1 m [35].…”

High‐resolution aerial infrared mapping of groundwater discharge to the coastal ocean

“…TIR data are thus best collected when cloud cover is minimal, seas are tranquil, and tide is low (Urish and McKenna 2004), maximizing groundwater flow and signal. For example, we collected data when there was a 0% chance of precipitation, cloud cover was forecasted to be < 20%, wave height was typically < 0.6 m, 0.3 m was better (calm conditions for the study area), and tide was ebbing to the lowest-low tide of the day.…”

Hydrogeology and hydrology of the Punta Cabullones wetland area, Ponce, southern Puerto Rico, 2007-08

“…The only distinctive spatial pattern in the K variation is that the lowest values are those for the auger holes nearest to the shore line. Values of K of similar orders of magnitude were obtained for beach sands in Cape Cod, Massachusetts, from the analysis of tidal load effects (Urish and McKenna, 2004).…”