downspouts, sump pumps, and unsealed manhole covers, or infiltration of stormwater via leaks in underground pipes, valves, and connections can cause peak flows
which may be many times the typical dry weather flows [31–33]. During dry weather
periods, collection systems may interact with groundwater, experiencing net infiltration or, alternatively, exfiltration [34], depending on the level of the water table in
relation to the conveyance pipes. Inflow and infiltration (I/I), which is in part
associated with deteriorating infrastructure, is an important and costly problem for
the wastewater industry because it can cause sanitary sewer overflows that affect
water quality in receiving streams. Also, any additional flow must be accounted for in
the design capacity of wastewater treatment plants.
In the initial stages of this project, an effort was made to estimate the “wet
weather” portion of I/I so that it could be accounted for in the estimate of PWS
consumptive use from withdrawal and discharge data. Simple linear regressions
demonstrated that monthly WWTP discharges from municipal facilities increase
significantly with monthly precipitation. However, further analyses, using multiple
regressions of monthly discharge on total monthly precipitation and on season,
indicated that discharges are also strongly correlated with season. Results for two
upper Potomac communities are plotted in Fig. 3. The graphs show that mean
monthly discharge and estimated “dry weather discharge” both have a seasonal
variation, with discharges highest in the winter months (December, January, and
February) and lowest in the summer months (June, July, and August). Dry weather
discharge is the estimated discharge not attributable to precipitation. In the examples shown, all model coefficients were significant at the 95 % confidence level.
Note that without I/I, discharges would be expected to be relatively constant,
reflective of indoor water use rates, and to be less than withdrawals, reflecting
consumptive and nonconsumptive losses. The seasonal dependence evident in
Fig. 3 can be attributed to the variation in dry weather infiltration/exfiltration into
wastewater collection pipes due to fluctuating groundwater levels, which are
significantly higher in the upper Potomac Basin in the winter than in the summer.
These analyses suggest that monthly discharge data from municipal WWTPs
cannot be used to obtain estimates of consumptive use for Potomac Basin communities because water use return flows are comingled with wet weather inflows and
groundwater infiltration/exfiltration. Though it might be feasible in some cases to
estimate and account for the wet weather component of discharge, no procedure is
currently available to estimate the effects of dry weather infiltration or exfiltration
that appears to occur due to fluctuating groundwater levels.
Because municipal WWTP discharge data is not reflective of water use return
flows, an alternative method for estimating consumptive use, the winter base rate
(WBR) method, is often used [22, 35–38]. The method is applicable in regions with
a temperate climate where it is reasonable to assume that consumptive use is
primarily due to outdoor water use in non-winter months. This method does not
rely on WWTP discharge data, but rather estimates consumptive use solely from
monthly water withdrawal data. The winter base rate method is based on the
following assumptions: (1) no significant consumptive use occurs in the winter
months; and (2) the observed difference between wintertime withdrawals and
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J. Ducnuigeen et al.
which may be many times the typical dry weather flows [31–33]. During dry weather
periods, collection systems may interact with groundwater, experiencing net infiltration or, alternatively, exfiltration [34], depending on the level of the water table in
relation to the conveyance pipes. Inflow and infiltration (I/I), which is in part
associated with deteriorating infrastructure, is an important and costly problem for
the wastewater industry because it can cause sanitary sewer overflows that affect
water quality in receiving streams. Also, any additional flow must be accounted for in
the design capacity of wastewater treatment plants.
In the initial stages of this project, an effort was made to estimate the “wet
weather” portion of I/I so that it could be accounted for in the estimate of PWS
consumptive use from withdrawal and discharge data. Simple linear regressions
demonstrated that monthly WWTP discharges from municipal facilities increase
significantly with monthly precipitation. However, further analyses, using multiple
regressions of monthly discharge on total monthly precipitation and on season,
indicated that discharges are also strongly correlated with season. Results for two
upper Potomac communities are plotted in Fig. 3. The graphs show that mean
monthly discharge and estimated “dry weather discharge” both have a seasonal
variation, with discharges highest in the winter months (December, January, and
February) and lowest in the summer months (June, July, and August). Dry weather
discharge is the estimated discharge not attributable to precipitation. In the examples shown, all model coefficients were significant at the 95 % confidence level.
Note that without I/I, discharges would be expected to be relatively constant,
reflective of indoor water use rates, and to be less than withdrawals, reflecting
consumptive and nonconsumptive losses. The seasonal dependence evident in
Fig. 3 can be attributed to the variation in dry weather infiltration/exfiltration into
wastewater collection pipes due to fluctuating groundwater levels, which are
significantly higher in the upper Potomac Basin in the winter than in the summer.
These analyses suggest that monthly discharge data from municipal WWTPs
cannot be used to obtain estimates of consumptive use for Potomac Basin communities because water use return flows are comingled with wet weather inflows and
groundwater infiltration/exfiltration. Though it might be feasible in some cases to
estimate and account for the wet weather component of discharge, no procedure is
currently available to estimate the effects of dry weather infiltration or exfiltration
that appears to occur due to fluctuating groundwater levels.
Because municipal WWTP discharge data is not reflective of water use return
flows, an alternative method for estimating consumptive use, the winter base rate
(WBR) method, is often used [22, 35–38]. The method is applicable in regions with
a temperate climate where it is reasonable to assume that consumptive use is
primarily due to outdoor water use in non-winter months. This method does not
rely on WWTP discharge data, but rather estimates consumptive use solely from
monthly water withdrawal data. The winter base rate method is based on the
following assumptions: (1) no significant consumptive use occurs in the winter
months; and (2) the observed difference between wintertime withdrawals and
152
J. Ducnuigeen et al.