From submarine to lacustrine groundwater discharge

J Lewandowski,G Nützmann,D O Rosenberry,F Pöschke,K Meinikmann

doi:10.5194/piahs-365-72-2015

Abstract

Abstract. Submarine groundwater discharge (SGD) and its role in marine nutrient cycling are well known since the last decade. The freshwater equivalent, lacustrine groundwater discharge (LGD), is often still disregarded, although first reports of LGD are more than 50 years old. We identify nine different reasons why groundwater has long been disregarded in both freshwater and marine environments such as invisibility of groundwater discharge, the size of the interface and its difficult accessibility. Although there are some fundamental differences in the hydrology of SGD and LGD, caused primarily by seawater recirculation that occurs only in cases of SGD, there are also a lot of similarities such as a focusing of discharge to near-shore areas. Nutrient concentrations in groundwater near the groundwater–surface water interface might be anthropogenically enriched. Due to spatial heterogeneity of aquifer characteristics and biogeochemical processes, the quantification of groundwater-borne nutrient loads is challenging. Both nitrogen and phosphorus might be mobile in near-shore aquifers and in a lot of case studies large groundwater-borne nutrient loads have been reported.

Highlights

Groundwater exfiltration into marine systems, is called submarine groundwater discharge (SGD) in the literature and defined as “any and all flow of water on continental margins from the seabed to the coastal ocean, regardless of the fluid composition or driving force” (Burnett et al 2003)
(d) Piezometers installed at a single point but at variable depths in the lake bed, piezometers installed along a line perpendicular to the shoreline, or installation of several groundwater observation wells in the catchment provide data allowing lacustrine groundwater discharge (LGD) rates to be calculated from hydraulic gradients based on Darcy’s law
The airborne study of Submarine groundwater discharge (SGD) with thermal infrared (TIR) imaging is much easier in marine settings since the fresher, less dense groundwater will always float on top of the sea water, while a complicated set of prerequisites is required in limnetic settings (Lewandowski et al 2013)

Summary

INTRODUCTION

Groundwater exfiltration into marine systems, is called submarine groundwater discharge (SGD) in the literature and defined as “any and all flow of water on continental margins from the seabed to the coastal ocean, regardless of the fluid composition or driving force” (Burnett et al 2003). (f) Spatial heterogeneity of discharge rates and groundwater composition result in large numbers of measurements required for reliable determinations of the LGD component in budgets. Lakes in contact with more than one aquifer may have large groundwater discharge rates; i.e. in addition to near-shore areas LGD can occur directly below the depth where the aquitard separating the two aquifers intersects the lake. (d) Piezometers installed at a single point but at variable depths in the lake bed (vertical flow), piezometers installed along a line perpendicular to the shoreline (horizontal flow), or installation of several groundwater observation wells in the catchment provide data allowing LGD rates to be calculated from hydraulic gradients based on Darcy’s law. (e) LGD can be determined with injection of artificial tracers (e.g. salt, fluorescent dye)

Integrating methods:

Methods used to quantify nutrient fluxes

Findings

CONCLUSION AND OUTLOOK