COASTAL
FRINGE
GROUNDWATER:
CONSIDERATIONS
FOR
ENVIRONMENTAL MONITORING WELL DESIGN, COASTAL DISCHARGE
SAMPLING & COASTAL CHEMISTRY ASSESSMENT
Aslan Perwick,1 Alan Pattle,1
1
Pattle Delamore Partners Ltd
Aims & Introduction
-
To understand the groundwater hydraulics of the coastal fringe and its implications for
environmental groundwater chemistry assessment
-
To create good groundwater monitoring network design (well and spring sampling) within
the coastal fringe setting which targets contaminant mass flux
-
Use basic geochemical modelling to determine groundwater – sea water mixing and
reaction processes
Fundamental Principals
The coastal fringe (i.e. <100 m from coast) can be a difficult place to design a groundwater
monitoring and sampling framework, and equally as difficult to determine groundwater chemistry
results.
The influence of; tidal processes, vertical groundwater flow gradients, density
differences, recent climatic factors and marine water chemistry mixing and reactions, can greatly
impact the groundwater quality results in coastal fringe monitoring wells and spring samples if
good groundwater science is not employed.
This presentation will discuss the groundwater hydraulics within the coastal fringe for a variety of
common NZ coastal settings and will describe key considerations for those who are planning
contaminated groundwater investigations and/or monitoring schemes within the coastal fringe.
Topics covered will include; options for placing well screens, best position(s) to collect beach
face/foreshore groundwater discharge samples, timing of sampling within the tidal cycle, common
reactions between groundwater and saline water – all based on targeting contaminant mass flux.
The use of basic geochemical modelling to interpret water chemistry observations and the
impacts of marine water mixing using a case-study will also be presented.
Implementation Considerations
Groundwater flow paths within the coastal margin contain a significant element of vertical
movement – both from the forces of freshwater-saline water density differences, and tidal forces.
The reliability of groundwater chemistry results from monitoring wells within the coastal fringe can
be significantly improved, with respect to targeting contaminant mass flux, through good design
based on understanding of the hydraulics at play within a particular coastal margin.
Beach face/foreshore water quality sampling can provide highly useful data on the quality of
groundwater discharge and should be included in a coastal fringe groundwater assessment – but
sampling location, timing and consideration of recent climatic factors is key to a meaningful
assessment. The figure below (Urish and McKenna, 2004), aides in displaying where and when
along a beach face/foreshore is best to collect a groundwater discharge sample in a sand
dominated coastal fringe: mid-lower beach face, within 1 hour either side of low-tide. Sampling
too high up the beach face will be influenced by saline water from the preceding flood tide, and
should be avoided.
Figure Beach and groundwater discharge concepts (Urish and McKenna, 2004)
Many other elements require consideration for both well design and sample event planning,
namely; rainfall, wind, swell and tidal patterns, as well as seasonal effects – which can all
influence the reliability / quality of groundwater chemistry sample within the coastal fringe.
The use of basic PHREEQC reactive geochemical modelling along with understanding of coastal
groundwater hydraulics has been used at an unnamed site to quantify the impacts of groundwater
mixing with natural saline water within a coastal fringe zone. Precipitation of carbonate minerals
and subsequent changes to groundwater contaminant chemistry is used as a prime example of
the impact coastal fringe process can have on groundwater quality.
References
Urish, D. & McKenna, T., 2004. Tidal effects on ground water discharge through a sandy marine
beach. Ground Water - Oceans Issue, 42(7), pp. 971-982.