7 research outputs found
Deconstructing Demand: The Anthropogenic and Climatic Drivers of Urban Water Consumption
Cities in drought
prone regions of the world such as South East
Australia are faced with escalating water scarcity and security challenges.
Here we use 72 years of urban water consumption data from Melbourne,
Australia, a city that recently overcame a 12 year “Millennium
Drought”, to evaluate (1) the relative importance of climatic
and anthropogenic drivers of urban water demand (using wavelet-based
approaches) and (2) the relative contribution of various water saving
strategies to demand reduction during the Millennium Drought. Our
analysis points to conservation as a dominant driver of urban water
savings (69%), followed by nonrevenue water reduction (e.g., reduced
meter error and leaks in the potable distribution system; 29%), and
potable substitution with alternative sources like rain or recycled
water (3%). Per-capita consumption exhibited both climatic and anthropogenic
signatures, with rainfall and temperature explaining approximately
55% of the variance. Anthropogenic controls were also strong (up to
45% variance explained). These controls were nonstationary and frequency-specific,
with conservation measures like outdoor water restrictions impacting
seasonal water use and technological innovation/changing social norms
impacting lower frequency (baseline) use. The above-noted nonstationarity
implies that wavelets, which do not assume stationarity, show promise
for use in future predictive models of demand
First-Order Contaminant Removal in the Hyporheic Zone of Streams: Physical Insights from a Simple Analytical Model
A simple
analytical model is presented for the removal of stream-borne
contaminants by hyporheic exchange across duned or rippled streambeds.
The model assumes a steady-state balance between contaminant supply
from the stream and first-order reaction in the sediment. Hyporheic
exchange occurs by bed form pumping, in which water and contaminants
flow into bed forms in high-pressure regions (downwelling zones) and
out of bed forms in low-pressure regions (upwelling zones). Model-predicted
contaminant concentrations are higher in downwelling zones than upwelling
zones, reflecting the strong coupling that exists between transport
and reaction in these systems. When flow-averaged, the concentration
difference across upwelling and downwelling zones drives a net contaminant
flux into the sediment bed proportional to the average downwelling
velocity. The downwelling velocity is functionally equivalent to a
mass transfer coefficient, and can be estimated from stream state
variables including stream velocity, bed form geometry, and the hydraulic
conductivity and porosity of the sediment. Increasing the mass transfer
coefficient increases the fraction of streamwater cycling through
the hyporheic zone (per unit length of stream) but also decreases
the time contaminants undergo first-order reaction in the sediment.
As a consequence, small changes in stream state variables can significantly
alter the performance of hyporheic zone treatment systems
Bedforms as Biocatalytic Filters: A Pumping and Streamline Segregation Model for Nitrate Removal in Permeable Sediments
Bedforms
are a focal point of carbon and nitrogen cycling in streams
and coastal marine ecosystems. In this paper, we develop and test
a mechanistic model, the “pumping and streamline segregation”
or PASS model, for nitrate removal in bedforms. The PASS model dramatically
reduces computational overhead associated with modeling nitrogen transformations
in bedforms and reproduces (within a factor of 2 or better) previously
published measurements and models of biogeochemical reaction rates,
benthic fluxes, and in-sediment nutrient and oxygen concentrations.
Application of the PASS model to a diverse set of marine and freshwater
environments indicates that (1) physical controls on nitrate removal
in a bedform include the pore water flushing rate, residence time
distribution, and relative rates of respiration and transport (as
represented by the Damkohler number); (2) the biogeochemical pathway
for nitrate removal is an environment-specific combination of direct
denitrification of stream nitrate and coupled nitrification-denitrification
of stream and/or sediment ammonium; and (3) permeable sediments are
almost always a net source of dissolved inorganic nitrogen. The PASS
model also provides a mechanistic explanation for previously published
empirical correlations showing denitrification velocity (N<sub>2</sub> flux divided by nitrate concentration) declines as a power law of
nitrate concentration in a stream (Mulholland et al. Nature, 2008, 452, 202−205)
Predictive Power of Clean Bed Filtration Theory for Fecal Indicator Bacteria Removal in Stormwater Biofilters
Green
infrastructure (also referred to as low impact development,
or LID) has the potential to transform urban stormwater runoff from
an environmental threat to a valuable water resource. In this paper
we focus on the removal of fecal indicator bacteria (FIB, a pollutant
responsible for runoff-associated inland and coastal beach closures)
in stormwater biofilters (a common type of green infrastructure).
Drawing on a combination of previously published and new laboratory
studies of FIB removal in biofilters, we find that 66% of the variance
in FIB removal rates can be explained by clean bed filtration theory
(CBFT, 31%), antecedent dry period (14%), study effect (8%), biofilter
age (7%), and the presence or absence of shrubs (6%). Our analysis
suggests that, with the exception of shrubs, plants affect FIB removal
indirectly by changing the infiltration rate, not directly by changing
the FIB removal mechanisms or altering filtration rates in ways not
already accounted for by CBFT. The analysis presented here represents
a significant step forward in our understanding of how physicochemical
theories (such as CBFT) can be melded with hydrology, engineering
design, and ecology to improve the water quality benefits of green
infrastructure
Small Drains, Big Problems: The Impact of Dry Weather Runoff on Shoreline Water Quality at Enclosed Beaches
Enclosed
beaches along urban coastlines are frequent hot spots
of fecal indicator bacteria (FIB) pollution. In this paper we present
field measurements and modeling studies aimed at evaluating the impact
of small storm drains on FIB pollution at enclosed beaches in Newport
Bay, the second largest tidal embayment in Southern California. Our
results suggest that small drains have a disproportionate impact on
enclosed beach water quality for five reasons: (1) dry weather surface
flows (primarily from overirrigation of lawns and ornamental plants)
harbor FIB at concentrations exceeding recreational water quality
criteria; (2) small drains can trap dry weather runoff during high
tide, and then release it in a bolus during the falling tide when
drainpipe outlets are exposed; (3) nearshore turbulence is low (turbulent
diffusivities approximately 10<sup>–3</sup> m<sup>2</sup> s<sup>–1</sup>), limiting dilution of FIB and other runoff-associated
pollutants once they enter the bay; (4) once in the bay, runoff can
form buoyant plumes that further limit vertical mixing and dilution;
and (5) local winds can force buoyant runoff plumes back against the
shoreline, where water depth is minimal and human contact likely.
Outdoor water conservation and urban retrofits that minimize the volume
of dry and wet weather runoff entering the local storm drain system
may be the best option for improving beach water quality in Newport
Bay and other urban-impacted enclosed beaches
DataSheet1_Freshwater salinization syndrome limits management efforts to improve water quality.pdf
Freshwater Salinization Syndrome (FSS) refers to groups of biological, physical, and chemical impacts which commonly occur together in response to salinization. FSS can be assessed by the mobilization of chemical mixtures, termed “chemical cocktails”, in watersheds. Currently, we do not know if salinization and mobilization of chemical cocktails along streams can be mitigated or reversed using restoration and conservation strategies. We investigated 1) the formation of chemical cocktails temporally and spatially along streams experiencing different levels of restoration and riparian forest conservation and 2) the potential for attenuation of chemical cocktails and salt ions along flowpaths through conservation and restoration areas. We monitored high-frequency temporal and longitudinal changes in streamwater chemistry in response to different pollution events (i.e., road salt, stormwater runoff, wastewater effluent, and baseflow conditions) and several types of watershed management or conservation efforts in six urban watersheds in the Chesapeake Bay watershed. Principal component analysis (PCA) indicates that chemical cocktails which formed along flowpaths (i.e., permanent reaches of a stream) varied due to pollution events. In response to winter road salt applications, the chemical cocktails were enriched in salts and metals (e.g., Na+, Mn, and Cu). During most baseflow and stormflow conditions, chemical cocktails were less enriched in salt ions and trace metals. Downstream attenuation of salt ions occurred during baseflow and stormflow conditions along flowpaths through regional parks, stream-floodplain restorations, and a national park. Conversely, chemical mixtures of salt ions and metals, which formed in response to multiple road salt applications or prolonged road salt exposure, did not show patterns of rapid attenuation downstream. Multiple linear regression was used to investigate variables that influence changes in chemical cocktails along flowpaths. Attenuation and dilution of salt ions and chemical cocktails along stream flowpaths was significantly related to riparian forest buffer width, types of salt pollution, and distance downstream. Although salt ions and chemical cocktails can be attenuated and diluted in response to conservation and restoration efforts at lower concentration ranges, there can be limitations in attenuation during road salt events, particularly if storm drains bypass riparian buffers.</p
From Rain Tanks to Catchments: Use of Low-Impact Development To Address Hydrologic Symptoms of the Urban Stream Syndrome
Catchment urbanization perturbs the
water and sediment budgets
of streams, degrades stream health and function, and causes a constellation
of flow, water quality, and ecological symptoms collectively known
as the urban stream syndrome. Low-impact development (LID) technologies
address the hydrologic symptoms of the urban stream syndrome by mimicking
natural flow paths and restoring a natural water balance. Over annual
time scales, the volumes of stormwater that should be infiltrated
and harvested can be estimated from a catchment-scale water-balance
given local climate conditions and preurban land cover. For all but
the wettest regions of the world, a much larger volume of stormwater
runoff should be harvested than infiltrated to maintain stream hydrology
in a preurban state. Efforts to prevent or reverse hydrologic symptoms
associated with the urban stream syndrome will therefore require:
(1) selecting the right mix of LID technologies that provide regionally
tailored ratios of stormwater harvesting and infiltration; (2) integrating
these LID technologies into next-generation drainage systems; (3)
maximizing potential cobenefits including water supply augmentation,
flood protection, improved water quality, and urban amenities; and
(4) long-term hydrologic monitoring to evaluate the efficacy of LID
interventions