Empirical relationships of groundwater head–salinity response to variations of sea level and vertical recharge in coastal confined karst aquifers

Abstract

Coastal aquifers are characterized by a mixing zone with freshwater-saltwater interactions, which have a strong relationship with hydrological forcings such as astronomical and storm tides, aquifer recharge and pumping effects. These forcings govern the aquifer hydraulic head, the spatial distribution of groundwater salinity and the saline interface position. This work is an empirical evaluation through time-series analysis between aquifer head and groundwater salinity associated with the sea-level dynamics and the aquifer recharge. Groundwater pressure, temperature and salinity were measured in a confined aquifer in the northwest coast of Yucatan (México) during May 2017-May 2018, along with precipitation. Cross-correlation and linear Pearson correlation (r) analyses were performed with the data time series, separating astronomical and meteorological tides and vertical recharge effects. The results show that the astronomical and meteorological tides are directly correlated with the aquifer head response (0.71 < r < 0.99). Salinity has a direct and strong relationship with the astronomical tide (0.76 < r < 0.98), while the meteorological tide does not (r < 0.5). The vertical recharge showed a moderate correlation with the aquifer head (0.5 < r < 0.7) and a nonsignificant correlation with the groundwater salinity (r < 0.5). In this study, the sea level (r > 0.7) is a more important forcing than the vertical recharge (with 0.5 < r < 0.7). Empirical relationships through time-series analysis and the separation of individual hydrological forcings in the analysis are powerful tools to study, define and validate the conceptual model of the aquifer.

On the Understanding of the Hydrodynamics and the Causes of Saltwater Intrusion on Lagoon Tidal Springs

“…Villasuso [37] shows the existence of a low permeability layer of sand limestones inland, which can act as a semi-confining layer that extends further inland. Similar results are reported by Canul-Macario et al [36], where this layer extends more than 12 km from the coast at Sisal. Given its karstic nature, the aquifer is highly vulnerable to surficial contamination [33,39,40].…”

“…Moreover, the effect of the confinement can also be seen in Figure 2c, where signals of wells P5 and B9 (Figure 1b), located further in land, present a tidal response and also show a strong correlation with the sea and less or similar attenuation than the stations L1 and L2 in the lagoon. This suggest that the water level in the spring pool is highly controlled by the confined aquifer, as reported by [18,36,37], and its interaction with the ocean tide. In fact, White et al [64] show that the attenuation of the tide in confined aquifers is low and the effects extend more than in unconfined aquifers, which was also found by Canul-Macario et al [36] and Medina [67] in our study area.…”

“…Tertiary deposits are composed mainly of limestone deposits from the Carrillo Puerto formation [34,35]. The aquifer consists of a lens of fresh water under-laid by saline water [18,32,33], the depth of the saline interface on well P5 is about 15 m and increases landwards [36]. The regional flow is from the southeast to the northeast, toward the coast, and shows a low hydraulic gradient (of the order of 10 −2 m/km [14]).…”

“…where s X , s Y are the sample variances of X and Y, respectively [63]. This analysis was successfully employed in Canul-Macario [36] to study the tide propagation in the aquifer.…”

Regional Hydrogeochemical Evolution of Groundwater in the Ring of Cenotes, Yucatán (Mexico): An Inverse Modelling Approach

“…This higher seawater mixing towards Celestun relative to Dzilam Bravo could be related to hydrogeological coastal differences among these two sites. Along the Yucatán's coast, the confinement influences the interaction between seawater and the aquifer, affecting the coastal aquifer's salinity [25,44,45]. The coastal confinement effect extends about 20 km towards Celestun, but only about 3 km toward Dzilam Bravo [24,46].…”