The Influence of Tidal Marshes on Upland Groundwater Discharge to Estuaries
Judson W. Harvey; William E. Odum
Biogeochemistry, Vol. 10, No. 3, Groundwater Inputs to Coastal Waters. (Aug., 1990), pp.
217-236.
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Biogeockemistq);
10:
21
7-236,
1990
0
1990
Kluwer Academic Publishers. Printed in the Netherlands
The influence of tidal marshes on upland groundwater
discharge to estuaries
JUDSON W. HARVEY
&
WILLIAM
E.
ODUM
Department of Environmental Sciences, Clark Hall, Univer~ity of Virginia, Clzarlottesville, VA
22903,
USA
Key
words:
groundwater, hydrology, pore water chemistry, salt marshes, solute transport, tidal
freshwater marshes
Ahstract.
We investigated subsurface hydrology in two fringing tidal marshes and in underlying
aquifers in the coastal plain of Virginia. Vertical distributions of hydraulic conductivity, hydraulic
head and salinity were measured in each marsh and a nearby subtidal sediment. Discharge of
hillslope groundwater into the base of the marshes and subtidal sediment was calculated using
Darcy's law. In the marshes, fluxes of pore water across the sediment surface were measured or
estimated by water balance methods. The vertical distribution of salt in shoreline sediments was
modeled to assess transport and mixing conditions at depth.
Hydraulic gradients were upward beneath shoreline sediments: indicating that groundwater was
passing through marsh and
subtidal deposits before reaching the estuary. Calculated discharge
(6
to 10 liters per meter of shoreline per day) was small relative to fluxes of pore water across the marsh
surface at those sites; even where discharge was maximal (at the upland border) it was 10 to 50 times
less than infiltration into marsh soils. Pore water turnover in our marshes was therefore dominated
by exchange with estuarine surface water. In contrast, new interstitial water entering
subtidal
sediments appeared to be primarily groundwater, discharged from below.
The presence of fringing tidal marshes delayed transport and increased mixing of groundwater
and solute as it traveled towards the estuaries. Soil-contact times of discharged groundwater were
up to 100% longer in marshes than in
subtidal shoreline sediments. Measured and modeled salinity
profiles indicated that, prior to export to estuaries, the solutes of groundwater, marsh pore water
and estuarine surface water were more thoroughly mixed in marsh soils compared to
subtidal
shoreline sediments. These findings suggest that transport of reactive solutes in groundwater may
be strongly influenced by shoreline type. Longer soil-contact times in marshes provide greater
opportunity for immobilization of excess nutrients by plants, microbes and by adsorption on
sediment. Also, the greater dispersive mixing of groundwater and pore water in marshes should lead
to increased availability of labile, dissolved organic carbon at depth which could in turn enhance
microbial activity and increase the rate of denitrification in situations where groundwater nitrate is
high.
Introduction
Groundwater discharge to estuarine waters has been a topic of theoretical and
practical interest for at least a century (reviewed in Freeze
&
Cherry 1979). Most
work has stressed controls on seawater intrusion to freshwater aquifers (e.g.
Henry 1960; Cooper et al. 1964) and it has been only recently that field measure-
ments of groundwater discharge have shown the importance of subsurface flow
on water and nutrient budgets in estuaries. For example, upland aquifers
contribute to direct groundwater discharge greater than 20% of the freshwater
and 75% of the nitrogen that enters Great South Bay, New York (Bokuniewicz
1980; Capone
&
Bautista 1985). Excess nitrogen in groundwater derived from
sewage and fertilizer has drastically affected water quality in other estuaries and
coastal lagoons as well (Johannes
&
Hearn 1985).
In the Mid-Atlantic and Northeast United States, groundwater discharge to
estuaries has been shown to be focused at the shoreline in a zone typically
extending 30 to 100 m offshore (Lee 1980; Valiela et al. 1980; Bokuniewicz 1980;
Capone
&
Bautista 1985). Most studies in estuaries have been conducted on
permanently flooded (subtidal) estuarine shorelines; fewer studies have inves-
tigated upland groundwater discharge along shorelines where intertidal marshes
are present. Intertidal marshes comprise 40% of the area of coastal lagoon
basins in the eastern United States
(Hayden
&
Dolan 1979). The marshes are
often present at upland borders where they have maximal potential to interact
with discharging groundwater from hillslope aquifers. Yet the effect of tidal
marshes, either on the magnitude of groundwater or upon the fate of dissolved
constituents being transported with the groundwater, is poorly understood.
We studied groundwater discharge from upland aquifers into two marshes on
the coastal plain of Virginia. Our investigations had a dual purpose: first, to
examine controls on the magnitude of fluxes, including stratigraphy and
hydraulic properties of marsh soils and underlying aquifers; second to assess the
fate of groundwater discharged to the marsh. We began by formulating mass
balance equations for pore water and solute for each shoreline type. Next, pore
water and solute budgets were developed from field data, and pore water
salinities from marshes and
subtidal shorelines were compared with profiles
generated by a solute transport model. Subsurface fluxes, residence times and
the degree of dispersive mixing of groundwater and marsh pore water were
estimated and compared between marsh and subtidal situations.
Site description and methods
The Mid-Atlantic coastal plain from New Jersey to North Carolina is composed
of sands and clays of Cretaceous age or younger that were deposited atop
bedrock in fluvial, estuarine, or marine environments (Back 1966). Stratigraphic
layers become thicker as they slope gently from west to east. Maximum thick-
ness of sediments are 200 to 400 m at the
Atlantlc Ocean shoreline. The general
movement of groundwater in the coastal plain is from recharge sites in the west
to discharge sites in the eastern rivers, estuaries and the Atlantic Ocean. Loc-
alized recharge and discharge occurs over the entire coastal plain. Figure 1 is a
schematic cross sectional view of two estuarine shorelines, with and without a
tidal marsh. Estuarine deposits have a high organic content and are of Holocene
age. Mineral materials of these deposits are clays with some reworked sands
from the underlying stratigraphic unit occurring at the base of the deposits;
marsh deposits are generally less than 5 m thick (Kraft et al. 1979). Underlying
aquifer
GW
Fig.
I.
Subsurface hydrological transport pathways at estuarine shorelines:
(a)
tidal marsh;
(b)
subtidal. The subsurface water fluxes are:
I,
infiltration flux;
G
W,
groundwater discharge flux;
ET,
evapotranspiration flux;
D,
pore water drainage flux. Note that only
D
and
G
W
occur if sediment
is subtidal, i.e. permanently flooded and unvegetated.
aquifers are of Pleistocene age or older and have very low organic content,
containing variable mixtures of sands and clays with some discontinuous layers
of nearly pure clay (Winner 1975; Kraft et al. 1979; Back 1966). In Virginia, the
water table aquifer is typically 3 to
20n1 thick and is underlain by the semi-
confining layers of a clayey-sand aquifer (Pliocene age) that contains fresh
interstitial water (Cederstrom 1943).
To quantify subsurface transport in shoreline environments it is useful to
distinguish between interstitial water of Holocene or younger deposits and the
water of the older, underlying aquifer. Interstitial water in shoreline sediments
(marsh soil or subtidal beach sediment) is referred to
aspore water
in this paper.
Interstitial water in the aquifer beneath shoreline sediments is referred to as
groundwater. Groundu~ater discharge
is defined as the surface transport of
groundwater from the underlying aquifer into the base of the marsh soil or
subtidal sediment (Fig. 1). Groundwater from below is the most likely source of
new pore water to a subtidal sediment; other possible pathways are by water
flow induced by bioturbation and bubble ebulliton. In tidal marsh soils,
injiltra-
tion
of flooding tidal water or precipitation is an additional source of water
input (Nuttle and Hemond 1988). Export of pore water from marsh soils occurs
by
pore water drainage
(Harvey et al. 1987) and by
evapotranspiration
(Dacey
&
Howes 1984).
Pore water balance
The net result of subsurface fluxes described above, averaged over many tidal
cycles, is a steady balance of subsurface inflows and outflows from a marsh soil;
,,)
positive and outflows by evapotranspiration (q
with inflows by groundwater discharge (q,,) and infiltration (q,) treated as
and drainage (4,) treated as
negative. The specific discharge (q) is defined by:
where the hydraulic conductivity
(K)
is a constant which expresses the effect of
water (e.g. viscosity) and marsh soil properties (e.g. permeability) on the rate at
which water can flow through soil for a given driving force. The driving force
is the gradient in hydraulic head
(dhldz) where hydraulic head (h) is a measure
of the potential energy per unit weight of water at a point in the flow system.
Hydraulic head has two components, pressure head and elevation head, ex-
pressed respectively as the pore pressure divided by the unit weight of water
(Plpg) and the elevation of the measurement above an arbitrary datum (z). In
marsh systems hydraulic head is measured as the height to which water rises in
a piezometer.
The pore water balance of
Eq.
(1) can be rearranged to find the net flux of
water across the sediment surface to the estuary and atmosphere. The difference
between surface influxes and effluxes yields a net flux across the surface equal
to the groundwater discharge rate.
Infiltration and evapotranspiration do not occur in
subtidal estuarine shoreli-
nes. If
Eq.
(1) is modified to reflect this difference, the net fluxes across sediment
surface is also found to equal the groundwater discharge.
Equations
(4)
and
(5)
indicate that
net
fluxes across the marsh or subtidal
sediment surface should equal the groundwater discharge rate, regardless of the
magnitude of the other fluxes. Away from creekbanks, the additional fluxes in
marsh soils
(I,
D,
and
ET)
do not contribute to a net, vertical advection of
