Linking Water Treatment with Wetland Restoration - Restorative Redevelopment of Devastated Ecocultural Landscapes - page 367

HYDROLOGY AND HYDRAULICS
Contaminant removal processes in wetlands are intimately connected to issues con-
cerning water movement and residence within the treatment cells. It is important to
define a few terms in this respect (Bays 2004). “Water regime” refers to the spatial
and temporal distribution of water elevations in a wetland; that is, how deep it is and
how depth is distributed throughout the system. “Hydro-period” refers to the number
of days per year or percentage of time that surface water is present in a given wetland
location. Because for treatment wetlands the strategy is to take advantage of the natu-
ral process that occurs in the sediments in a flooded condition, an objective becomes
to keep the system flooded year-round. But that may not necessarily be a sustainable
water regime for a given area, and it is possible that a desirable state may actually be
to cycle the hydro-period by varying it over time. “Flooding frequency” considers
how often the wetland floods. This is critically important for any natural system,
particularly those like the Iraqi wetlands that are (or rather, once were) dependent
on rivers carrying rain runoff. Today, the water regime in the Iraqi marshes has
been highly modified, with the once major spring freshet pulses being replaced by a
steady year-round flow of greatly diminished magnitude (France 2011).
The critically important part of wetland design, particularly in southern (hot)
regions, is a preliminary water balance calculated in order to ensure that an
adequate supply of water is present to maintain the restored wetlands. For wet-
lands in arid regions such as those in southern Iraq, the bottleneck for system
sustainability is of course the absence of rain during the summer. Enough water,
therefore, has to be present or be renewed frequently enough in order to sustain
the system, and usually this is determined by what inflow-outflow and evapora-
tion rates can be expected, as well as the leakiness of the soil in terms of its infil-
tration rate. The hydraulic loading rates for wetlands are normally in the order
of 1 to 10 cm per day. This is not a large amount of water to apply to a system,
which means when these numbers are extended out into large-scale applications,
a great deal of land is needed to treat the water. Given that land in southern Iraq
is not in short supply, Bays (2004) believes that prospects there look good from
this respect. The residence time for a system ranges anywhere from ten to thirty
days based upon the relationship of the volume to the flow. But in fact the issue
is that flow is not uniform in wetlands and because of this water balances are
often difficult to calculate, so much so that this topic represents a large part of
the current research efforts in terms of trying to understand how wetlands oper-
ate (Bays 2004).
The Valley Sanitation District Wetland in Indio, southern California, is, as men-
tioned, in the same kind of climatic regime as that in southern Iraq and in conse-
quence experiences an amazing loss of over 200 cm of evaporation per year which
absolutely controls the water balance (this particular artificial system is lined so
there are no groundwater losses). Indeed, with temperatures in excess of 35°C, these
wetlands can lose over 30 cm of water a month during the summer, such that it is
actually possible to measure the water levels receding on a daily basis. This type of
information can be used to estimate infiltration rates and evaporation rates in other
wetlands without bottom liners.