Neodymium Isotope Geochemistry of a Subterranean Estuary

Rare earth elements (REE) and Nd isotope compositions of surface and groundwaters from the Indian River Lagoon in Florida were measured to investigate the influence of submarine groundwater discharge (SGD) on these parameters in coastal waters. The Nd flux of the terrestrial component of SGD is around 0.7±0.03 μmol Nd/day per m of shoreline across the nearshore seepage face of the subterranean estuary. This translates to a terrestrial SGD Nd flux of 4±0.2 mmol/day for the entire 5,880 m long shoreline of the studied portion of the lagoon. The Nd flux from bioirrigation across the nearshore seepage face is 1±0.05 μmol Nd/day per m of shoreline, or 6±0.3 mmol/day for the entire shoreline. The combination of these two SGD fluxes is the same as the local, effective river water flux of Nd to the lagoon of 12.7±5.3 mmol/day. Using a similar approach, the marine-sourced SGD flux of Nd is 31.4±1.6 μmol Nd/day per m of shoreline, or 184±9.2 mmol/day for the investigated portion of the lagoon, which is 45 times higher than the terrestrial SGD Nd flux. Terrestrial-sourced SGD has an εNd(0) value of −5±0.42, which is similar to carbonate rocks (i.e., Ocala Limestone) from the Upper Floridan Aquifer (−5.6), but more radiogenic than the recirculated marine SGD, for which εNd(0) is −7±0.24. Marine SGD has a Nd isotope composition that is identical to the εNd(0) of Fe(III) oxide/oxyhydroxide coated sands of the surficial aquifer (−7.15±0.24 and −6.98±0.36). These secondary Fe(III) oxides/oxyhydroxides formed during subaerial weathering when sea level was substantially lower during the last glacial maximum. Subsequent flooding of these surficial sands by rising sea level followed by reductive dissolution of the Fe(III) oxide/oxyhydroxide coatings can explain the Nd isotope composition of the marine SGD component. Surficial waters of the Indian River Lagoon have an εNd(0) of −6.47±0.32, and are a mixture of terrestrial and marine SGD components, as well as the local rivers (−8.63 and −8.14). Nonetheless, the chief Nd source is marine SGD that has reacted with Fe(III) oxide/oxyhydroxide coatings on the surficial aquifer sands of the subterranean estuary.

The Peculiar Hydrology of West-Central Florida’s Sandhill Wetlands, Ponds, and Lakes – Part 2: Hydrogeologic Controls

This study investigates hydrogeologic controls on a peculiar, poorly studied type of geographically isolated wetland in west-central Florida, USA, locally referred to as “sandhill wetlands.” Their peculiarity lies in their connectivity to a large, regional aquifer, which controls their hydrology and influences their ecological expression. Six wetlands and one wetland-pond complex were examined using geophysical, lithologic, hydrologic, and ecological data. These data were used to configure site-specific hydrogeology, from which two conceptual models were developed. The first model depicts mechanisms of sandhill wetland connectivity to the regional aquifer. Three mechanisms of connectivity are proposed based on the degree and depth of aquifer confinement: 1) direct - due to wetland embedment directly in the unconfined regional aquifer; 2) indirect - due to embedment in a surficial aquifer, where groundwater exchange with the regional aquifer occurs through breaches in the semi-confining unit; and 3) none - due to embedment in a surficial aquifer where groundwater exchange with the regional aquifer does not occur because the semi-confining unit is too deep. The second model conceptualizes fundamental sandhill wetland ecohydrology. It depicts how the geomorphology of a sandhill depression relative to the range of the regional water table determine whether that feature will manifest as a wetland or as a pond, lake, sink, or upland. Findings from both models contribute to the limited understanding of sandhill wetland, pond, and lake ecohydrology and may be used to improve how they are classified, assessed, managed, and preserved as valuable natural resources.

Modelling of the Complex Groundwater Level Dynamics during Episodic Rainfall Events of a Surficial Aquifer in Southern Italy

We analyzed the complex dynamics that are involved the groundwater level variations due to the episodic rainfall supply in the Ionian coastal plain surficial aquifer located in Southern Italy. In this aquifer, as a consequence of the particular hydrogeological framework, both direct and lateral recharge mechanisms coexist. Hence, the dynamics of groundwater level variations are quite complex and strongly non-linear. Our focus was essentially on the short-term behavior of groundwater levels, with a specific analysis on episodic rainfall events. To model these dynamics, due to the presence of the preferential pathways in the infiltration processes, a kinematic dispersion wave model was used. Specifically, a one-dimensional and non-linear particle-based numerical model was developed. It uses ideal particles with constant water volume travel, according to celerity and hydraulic dispersion, to simulate the infiltration rate wave through the vadose zone. The infiltration rate that reaches the water table represents the input function to evaluate the aquifer groundwater level fluctuations. As a consequence of the special lithological and storage capacity characteristics of the surficial layers, groundwater flow conditions change from unconfined to confined. The developed model analyzes the direct groundwater supply under natural conditions, including episodic rainfall, and it has been validated using a high-resolution time series of rainfall data and groundwater level obtained from the monitoring station Terra Montonata.