Article pubs.acs.org/est
Elevated Dissolved Phosphorus in Riparian Groundwater along Gaining Urban Streams James W. Roy* and Greg Bickerton Canada Centre for Inland Waters, Environment Canada, Burlington, Ontario L7R 4A6, Canada S Supporting Information *
ABSTRACT: Findings of low concentrations of dissolved phosphorus in groundwater in large surveys [e.g., United States Geological Survey’s National Water-Quality Assessment (NAWQA) Program (Dubrovsky, N. M.; et al. The Quality of Our Nation’s Water: Nutrients in the Nation’s Streams and Groundwater, 1992−2004. U.S. Geological Survey Circular 1350; USGS: Reston, VA, 2010.); >5000 wells] support the common perception that groundwater is generally of little importance for transporting phosphorus. Here, we address whether this applies to urban riparian settings, where discharging groundwater may potentially contribute to urban stream syndrome and downstream eutrophication problems. This survey study includes 665 samples of groundwater collected along gaining stream reaches at six urban sites. Considering the combined sample set, 27% had soluble reactive phosphorus (SRP) concentrations >0.1 mg L−1, which is more than double that determined in the NAWQA Program (12%), while for individual sites the range was 12−52%, excluding one site with consistently low SRP (0%). None of the sites showed significant correlation between SRP and the artificial sweetener acesulfame, a promising wastewater indicator, including two with known wastewater contamination (but the lowest SRP). Rather, high SRP concentrations were associated with geochemically reducing conditions. This could mean that natural aquifer or stream sediment materials were a primary contributor of the elevated SRP observed in this study.
■
INTRODUCTION Elevated levels of nutrients have been associated with the degradation of ecological health of urban streams,2 commonly termed urban stream syndrome. Input of phosphorus (P) is of primary concern given its role in eutrophication,3 of the stream itself or downstream water bodies, and the potential subsequent effects of oxygen depletion and harmful algal blooms. Urbanization is known to increase the phosphorus release from watersheds.4 This has been attributed primarily to inputs from wastewater treatment plants, sewer discharges, and direct runoff from impervious infrastructure or areas under construction. The potential contribution of dissolved P from discharging groundwater to urban streams has not been actively studied. A recent report of findings from the U.S. Geological Survey’s (USGS) National Water-Quality Assessment (NAWQA) Program for nutrients in surface water and groundwater in the conterminous U.S. 1 is the largest published data compilation of groundwater P concentrations of which we are aware. It reported a background groundwater concentration of 0.03 mg L−1 as orthophosphate, derived from 166 wells from areas with minimal human development. In the complete nutrient data set of 5101 monitoring, domestic, and publicPublished 2014 by the American Chemical Society
supply wells, P concentrations were generally similar to background and varied little between 3 land-use categories: urban, agricultural, and major aquifers (much deeper wells). Although locally some concentrations were as high as 4.3 mg L−1, few (12%) of these samples had dissolved P concentrations >0.1 mg L−1, a value commonly associated with hypereutrophic surface waters.5 The USGS report concluded that dissolved P is “not persistent or mobile enough to affect groundwater concentrations significantly under most conditions.” The above study supports the common perception that groundwater is not an important vector for P. Indeed, most P compounds tend to precipitate or adsorb to soil and sediment materials,3 limiting transport with groundwater. However, P can be mobilized under certain geochemical conditions, such as high pH, high organic content, and geochemically reduced (anoxic) conditions,1,3,6,7 and when P retention sites in soils or sediments become saturated, generally following long periods of P application.8 Received: Revised: Accepted: Published: 1492
October 29, 2013 January 13, 2014 January 14, 2014 January 14, 2014 dx.doi.org/10.1021/es404801y | Environ. Sci. Technol. 2014, 48, 1492−1498
Environmental Science & Technology
Article
Table 1. Summary of Studies Reporting Elevated SRP Concentrations in Groundwater and Associated with a Variety of Potential Nutrient Sources or Land Usesa study
sampling method
maximum SRP (mg/L)
P source or land use
Roy and Malenica 201326 b Andres and Sims 201347 Wolf et al. 201239 Flores-Lòpez et al. 201115 b Qian et al. 201122 Palmer-Felgate et al. 201018 b Jalali 200916 Roy et al. 200921 b Jarvie et al. 200846 b Schilling and Jacobson 200811 b Vadas et al. 200714 b Banaszuk et al. 200510 b Robertson 200320 McCobb et al. 200317 b Carlyle and Hill 20016 b Kelly et al. 19999 b Spruill et al. 199827 Ptacek 199819 Driescher and Gelbrecht 199313 b Vanek 199324 b Shaw et al. 199025 b
drive points wells wells piezometers boreholes diffusion probes wells underwater seep diffusion probes wells wells wells, piezometers piezometers wells piezometers piezometers wells piezometers wells wells and seepage wells, peepers
0.81 1.1 2.3 0.96 11.6 42 2.6 0.7 5.2 1.3 12.3 3.1 4.8 4.6 0.95 2.9 1.4 1.5 10 11 1.3
urban; geologic materials? wastewater infiltration urban manure-treated fields urban sorption from septic waste? fertilizers; human waste? septic waste several possible sources geologic materials manure-treated fields geologic materials septic waste septic waste geologic materials geologic materials geologic materials septic waste sewage-treated fields fertilizers/human waste? geologic materials
a
In a few cases, values reported as dissolved orthophosphate are assumed to equal SRP. bDenotes study involving groundwater P discharge to a surface water body.
Table 2. Details on the Six Stream Sites and Overview of the Groundwater and Surface Water Results, Including Maximum Concentrations of Soluble Reactive Phosphorus (SRP) and the Artificial Sweetener Acesulfame (ace) surface watera
groundwater
Athabasca River (Jasper) Athabasca River (N Alberta) Dyment’s Creek (Barrie) Hewitt’s Creek (Barrie) Sackville River (Halifax) Zeke’s Brook (Greenwood) a
stream-side land uses
no. samples
SRP >0.1 mg/L (%)
max. SRP (mg/L)
max. ace (ng/L)
max. SRP (mg/L)
max. ace (ng/L)
camping, rail, forest (sewage) mining industry, forest residential, commercial (landfill) residential, wetlands (septics) commercial, residential residential, commercial, forest
102 31 189 84 126 133
0 48 52 12 20 23
0.034 1.1 1.5 0.22 0.44 0.67
3500 290 340 33 600 890 270
0.01 0.01 0.11 0.02 0.03 0.04
63 20 72 88 100 28
There were generally 1−3 surface water samples collected at each site.
therein) for shallow wetland sediments. However, such findings are not universal,30−32 especially in low-nutrient forested riparian zones.6,28 The range of findings encompassing the broad surveys and the site-specific studies indicates some uncertainty in the level of dissolved P that may be found in riparian groundwater in general. The objective of this study was to determine what levels of dissolved P are more representative of the concentrations expected in groundwater within the shallow sediments of gaining urban streams, for which there is little information currently available; i.e., the low levels generally found in the NAWQA well-based study1 or the elevated P levels reported in some studies with riparian and organic-rich sediments (Table 1). Through past studies, we have sampled shallow groundwater at 665 locations from gaining stream sections at 6 urban sites, several of which have known anthropogenic contaminant sources that may contribute P to groundwater. While P was rarely the focus of this past work, soluble reactive phosphorus (SRP) was analyzed, allowing for this retrospective investigation. Additionally, we discuss potential sources of P to these urban riparian groundwaters.
The NAWQA report1 did note some cases of naturally elevated dissolved P within aquifers, derived from organic9−11 or mineralogical12 sediments. Exceptional cases of high groundwater P concentrations have involved anthropogenic P sources including contamination from agricultural practices,13−16 domestic septic systems,17−21 and widespread urban pollution,22,23 attributed largely to leaking sewer systems. A list of studies reporting elevated P in groundwater is provided in Table 1, and yet, there is evidence that many such contaminant sources do not produce substantial groundwater P plumes due to immobilization or sorption processes.20 Few of the groundwater samples from the NAWQA study1 were collected from near-shore or stream riparian areas. In contrast, many of the studies reporting elevated P in groundwater involve groundwater discharge to surface water bodies, some with anthropogenic P sources,17,21,24 while others were associated with contributions from near-shore25,26 or nearstream (i.e., riparian)6,27 sediments. Net P release from riparian zones to their streams have been reported from several studies on riparian buffers.28 Elevated concentrations of dissolved P (mg L−1 range) have also been reported (ref 29 and references 1493
dx.doi.org/10.1021/es404801y | Environ. Sci. Technol. 2014, 48, 1492−1498
Environmental Science & Technology
■
Article
surpassed 0.1 mg L−1 SRP, the level associated with hypereutrophic surface water.5 However, at the remaining sites (Table 2), the percentage of samples with concentrations >0.1 mg L−1 ranged from 12% (Hewitt’s Creek) to 52% (Dyment’s Creek). Interestingly, the two sites with known wastewater influence had the lowest SRP levels, indicating that their inclusion in this survey of six stream sites has not biased the results toward higher SRP concentrations. Combining all 665 of the near-stream groundwater samples from the six sites, about 27% had concentrations >0.1 mg L−1, which is more than double the percentage level determined in the NAWQA study. In comparison, Roy and Malenica26 performed a similar survey of groundwater discharging along two ∼1 km lengths of urban shoreline. For that study, 24% of the 172 samples had SRP concentrations >0.1 mg L−1. Not only were the samples in this study generally elevated compared to background levels, but also some of the sites had samples with very high concentrations. At three sites, the maximum groundwater SRP concentrations reached 0.67, 1.1, and 1.5 mg L−1 (Table 2). A similar range of maximum SRP concentrations has been reported at other sites (Table 1), which are associated with several different P sources, including natural sediments. For some other studies, the maximum concentrations reported were even higher, especially for those addressing wastewater-impacted groundwater (e.g., 42 mg L−1 by Palmer-Felgate et al.;18 10 mg L−1 by Driescher and Gelbrecht;13 4.8 mg L−1 by Robertson;20 4.6 mg L−1 by McCobb et al.;17 Table 1). This study consisted of a survey of six urban sites within 3 Canadian provinces and a total of 665 samples of near-stream (riparian) groundwater collected along gaining reaches of a variety of different sized streams (or rivers). While not as extensive of a data set for P in groundwater as that of the USGS NAWQA Program survey,1 which tested >5000 wells across the conterminous US for nutrients, this is the first survey-type study to address specifically urban riparian groundwater. Its findings suggest that riparian groundwater in urban areas may commonly have higher SRP concentrations than is generally perceived as the norm or background level. However, the presence of elevated SRP in riparian groundwater should still be assessed on a site-by-site basis, especially in areas with grossly differing geology (e.g., exposed bedrock sites) than the sites in this study, which were predominantly composed of glacial or alluvial sediments (Table S1, Supporting Information). The SRP concentrations for the riparian groundwater at these six sites were generally more than an order of magnitude greater than their respective stream sample concentrations (Table 2). Therefore, discharging groundwater would be a potential source of SRP to these streams, although the temporal variability in both groundwater and stream SRP concentrations are unknown, and because they were collected at depths of >0.25 m below the streambed, they may not represent the nutrient concentrations of the groundwater discharging across the streambed interface. Phosphorus release to groundwater from surficial sediments could raise these SRP concentrations further, while various processes, such as sorption, mineral precipitation, and plant uptake could reduce them.29 Regardless, these study findings suggest that the groundwater contribution of P to urban streams should receive more attention for its potential role in urban stream syndrome, especially in influencing the epibenthic algal community,37 and in downstream eutrophication.
METHODS Geographic locations of the six urban sites are shown in Figure S1, Supporting Information. Detailed geological and mineralogical investigations of the sites were not made; however, surface geology of the general areas is listed in Table S1, Supporting Information. Of note, most samples were collected in unconsolidated sediments, commonly recent or past alluvium of the river valley, though some samples from the Athabasca River (northern Alberta) site were in loosely consolidated sandstone and limestone outcrops. The groundwater flow paths and recharge areas of these sites are not known. The surrounding land use for all sites (Table 2) includes at least some urban development (industrial, residential, and/or commercial), with known wastewater sources at 2 sites:33,34 residential septic systems for Hewitt’s Creek and a plume from infiltration of municipal wastewater (advanced nutrient treatment, which may limit P input somewhat) for Athabasca River (Jasper). At parts of Dyment’s Creek, an old landfill is present, but recent detailed investigation (unpublished) suggests it is not contributing P. The number of groundwater sample locations at each of the 6 stream sites is provided in Table 2. Samples were commonly spaced about 10−20 m apart, but larger separation distances apply to the Athabasca River (northern Alberta) site. Most were collected along one or both edges of the stream, except some midstream samples were taken for the Sackville River site. Given their proximity to the stream, we consider these as riparian groundwater samples, though we have no information on the flow paths and recharge areas. For several of the sites, data from multiple years have been combined. However, it is only for the 2 Barrie sites that there is some overlap of sampling areas, although this did not involve sampling from the exact same locations (as would occur for sampling from a fixed piezometer). The shallow groundwater sampling protocol generally follows that reported by Roy and Bickerton.35 Briefly, a removable mini-profiler with drive-point tip was driven vertically into the stream sediments using a hammer-drill. Groundwater at depths ranging between 0.25 and 1.5 m below the streambed was withdrawn using a peristaltic pump via a series of screened ports (∼12 cm section) on the drive-point. At each sample location, field measurements of pH, dissolved oxygen (DO), and electrical conductivity were made using hand-held meters prior to sampling. Groundwater was collected from only a single depth per location, where sufficient flow was obtained and field parameters differed substantially from those of the stream. In these past studies, the shallow groundwater samples were analyzed for numerous constituents, from basic geochemistry to various contaminants or potential tracers, including SRP. Complete details of sample preservation and the analytical methods for SRP and any additional constituents relevant to this study (e.g., major cations and anions, including nitrate, alkalinity, artificial sweeteners and anionic pesticides, ammonium) are provided by Roy and Bickerton.36
■
RESULTS AND DISCUSSION SRP Levels in Urban Riparian Groundwater. The riparian groundwater samples collected in this study were generally elevated in SRP compared to the USGS NAWQA background value of 0.03 mg L−1,1 at all of the sites except the Athabasca River, Jasper site. At this site, none of the samples 1494
dx.doi.org/10.1021/es404801y | Environ. Sci. Technol. 2014, 48, 1492−1498
Environmental Science & Technology
Article
SRP and the Wastewater Indicator Acesulfame. The artificial sweetener acesulfame is considered one of the best available tracers of wastewater in the environment,38 given its near-ubiquitous occurrence and general recalcitrance in wastewater, its low analytical detection limit, and the fact it is an anthropogenic compound. Acesulfame is also an anionic compound under most groundwater conditions and thus is expected to have limited tendency to sorb to aquifer solids. It has been found in groundwater in multiple countries [e.g., refs 34, 38, and 39]. High acesulfame concentrations were detected in groundwater samples at the two sites with known wastewater plumes, reaching 3500 and 34 000 ng L−1 at Hewitt’s Creek (residential septic systems) and the Athabasca River near Jasper (municipal sewage infiltration area), respectively. Meanwhile, the maximum concentrations in groundwater at the remaining sites ranged between 270 and 890 ng L−1 (Table 2), which are less than concentrations generally expected in the main body of wastewater plumes.34,40,41 Instead, these lower levels may represent acesulfame inputs to groundwater from municipal water (e.g., lawn watering; leaking pipes), which is often sourced by water bodies that may receive treated wastewater inputs. For example, Lake Simcoe near Barrie had acesulfame concentrations up to 275 ng L−1,26 but they may also reflect diffuse wastewater inputs from urban runoff or leaking sewer systems. Wolf et al.39 reported a maximum groundwater acesulfame concentration of 2870 ng L−1 in an urban area associated with leaking sewers. If elevated SRP concentrations in groundwater were associated with a wastewater source, then we might expect good correlation between SRP and acesulfame concentrations, although various flow and transport processes may act to separate the groundwater SRP from acesulfame, which is considered to behave as a conservative tracer in most conditions.38 For the Athabasca River, Jasper site, which has a known sewage plume but low SRP concentrations (Table 2), there was some correlation, but it was not overly strong (linear regression; R2 = 0.42). There was no correlation at Hewitt’s Creek (R2 = 0.03; not significant at 95% confidence level), despite the presence of septic system influences. Similarly, poor correlations were the norm at the remaining sites (Table S2, Supporting Information). These findings suggest that the SRP concentrations observed at most, if not all, of these sites are not a result of current or recent wastewater contamination, even including the two sites with known wastewater impacts on groundwater. At these two sites, P transport away from the wastewater source zone may have been limited by precipitation and sorption reactions, as has been noted for similar wastewater sources under certain geochemical conditions.20 However, it is also possible that the elevated SRP could result from desorption or dissolution of sediment-bound phosphorus from past wastewater inputs that no longer contain or never contained acesulfame (introduced to Canada about 2 decades ago). SRP and Other Constituents. Other water quality constituents that have been reported as sometimes being associated with groundwater SRP concentrations include alkalinity, calcium, ammonium, and pH.1 These potential relationships were also assessed for the six study sites using linear regression analysis. Only a few relationships at a few of the sites surpassed an R2 of 0.5 (Table S2, Supporting Information). The strongest relationship (R2 = 0.65) was with calcium at the Athabasca River, Jasper site. Here, the SRP concentrations were low, near common background levels,1 so
this relationship is of little interest for this study. The only other one of note (R2 = 0.62) was with ammonium at the Hewitt’s Creek site, which has known septic plume influences. Similar to SRP at this site, it seems unlikely that ammonium was supplied from the septic systems, given the poor correlation between SRP and acesulfame noted above, and a similarly poor correlation for ammonium and acesulfame (not shown). Instead, both nutrients at this site may be derived from natural sources, including the anaerobic degradation of organic matter, given that both compounds were more elevated in groundwater samples with dissolved oxygen
