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Groundwater or Floodwater? Assessing the Pathways of Metal Exports from a Coastal Acid Sulfate Soil Catchment Isaac R. Santos,*,† Jason de Weys,† and Bradley D. Eyre† †
Centre for Coastal Biogeochemistry, School of Environmental Science and Management, Southern Cross University, Lismore, NSW 2480, Australia
bS Supporting Information ABSTRACT: Daily observations of dissolved aluminum, iron, and manganese in an estuary downstream of a coastal acid sulfate soil (CASS) catchment provided insights into how floods and submarine groundwater discharge drive wetland metal exports. Extremely high Al, Fe, and Mn concentrations (up to 40, 374, and 8 mg L 1, respectively) were found in shallow acidic groundwaters from the Tuckean Swamp, Australia. Significant correlations between radon (a natural groundwater tracer) and metals in surface waters revealed that metal loads were driven primarily by groundwater discharge. Dissolved Fe, Mn, and Al loads during a 16-day flood triggered by a 213 mm rain event were respectively 80, 35, and 14% of the total surface water exports during the four months of observations. Counter clockwise hysteresis was observed for Fe and Mn in surface waters during the flood due to delayed groundwater inputs. Groundwater-derived Fe fluxes into artificial drains were 1 order of magnitude higher than total surface water exports, which is consistent with the known accumulation of monosulfidic black ooze within the wetland drains. Upscaling the Tuckean catchment export estimates yielded dissolved Fe fluxes from global acid sulfate soil catchments on the same order of magnitude of global river inputs into estuaries.
’ INTRODUCTION The relationship between riverine exports and biogeochemical processing of trace metals within estuaries has been studied in the last few decades.1 However, little is known about how floods and groundwater may contribute to trace metal cycling in estuaries. Since trace element concentrations in groundwater often exceed those in surface waters, groundwater can play a significant role in regional and global metal budgets.2 The contribution of groundwater to estuarine budgets will likely increase as human activity on coastal watersheds increases.3 Groundwater may be especially important at sites where low pH increases the solubility of metals. Millions of hectares of coastal floodplains and wetlands worldwide are underlain by sediments rich in iron sulfide minerals.4 When drained for agriculture and grazing, iron sulfide minerals oxidize and produce acid that can be flushed to adjacent waterways.5 In Australia and many other (sub-) tropical countries, thousands of kilometers of artificial drains have been constructed on coastal floodplains underlain by sulfidic soils.6 These drains are designed to lower the water table, but they also expose reducing soils to atmospheric oxygen. Previous studies have documented that low pH waters from coastal acid sulfate soils (CASS) are associated with trace metals concentrations exceeding water quality guidelines.7 Soluble metals from CASS, including Al3+ and Fe2+, can impact entire estuarine ecosystems8 and eventually be stored in coastal sediments.9 Episodic floods in tropical CASS catchments have been linked with severe estuarine degradation. During summer floods, floodplain r 2011 American Chemical Society
vegetation decomposition may lead to deoxygenation events that can result in fish kills.10 The discharge of acidic CASS groundwaters often follows the release of deoxygenated surface waters from the floodplain.11 There is a scarcity of research relating to the flux of heavy metals from CASS. Previous dissolved metal work in CASS catchments were largely based on spatial surveys5 or time series observations following flood events.12 To our knowledge, no studies have focused on the contribution of groundwater to surface water metal loads during a flood. In this paper, we investigate the contribution of groundwater to dissolved Al, Fe, and Mn exports from a subtropical CASS catchment into an estuarine embayment during a flood. We advance earlier investigations by (A) performing higher resolution, longer-term observations that captured contrasting hydrological conditions, and (B) linking groundwater discharge to surface water metal loads with concomitant radon, a natural groundwater tracer,13 measurements.
’ EXPERIMENTAL SECTION Experimental Site. Field observations were performed in the Tuckean Swamp in northern New South Wales, Australia (Figure 1). The swamp has been progressively cleared and drained to reduce periodic inundation, resulting in land suitable Received: July 25, 2011 Accepted: October 3, 2011 Revised: October 3, 2011 Published: October 03, 2011 9641
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Figure 1. The distribution of selected variables in Tuckean Swamp waterways in June 2009. The numbers adjacent to selected samples show concentrations. Radon and pH values from ref 11a. The gray scale on the upper left map represents the distribution of soils in the Tuckean Swamp according to maps from the Department of Land and Water.35 The dark gray area around the Main Drain represents areas with CASS at the ground surface. The larger light gray area represents areas with CASS within 1 m of the ground surface. The white area represents CASS absent or below 1 m below the ground surface. Squares represent the location of groundwater samples.
for grazing and agriculture.14 Large areas developed into CASS after drains were constructed. The drains range from roadside table drains 0.5 to 1 m deep to main drains up to 5 m deep and 25 m wide10a and are thought to enhance groundwater discharge and the associated release of metals from CASS.7,12 The Swamp drains into the Tuckean Broadwater, a major tidal tributary of the Richmond River estuary. Average annual rainfall is about 1800 mm with ∼65% falling from December to April.15 Low pH waters can be observed year-round in the Tuckean Broadwater and are related to
groundwater inputs.11a The floodplain is a large back barrier lagoon infilled with Quaternary sediments mostly near 1 m AHD. Complex sequences of marine sands and pyritic estuarine clays are overlain by alluvial sediments. Experimental Approach. Our approach consisted of (A) sampling groundwaters to characterize the endmember concentrations entering surface waters; (B) conducting a spatial survey within the Tuckean Swamp drains and natural creeks to identify possible groundwater points of entry; and (C) performing long9642
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Table 1. Average ((99% Confidence Interval) pH, Dissolved Oxygen, and Metal Concentrations in Groundwater Samplesa n
a
pH
DO (mg L 1)
Al (mg L 1)
Fe (mg L 1)
Mn (mg L 1)
all samples
31
5.54 ( 0.64
1.89 ( 0.46
5.5 ( 5.1
70 ( 46
1.6 ( 1.0
non-CASS samples (pH > 5)
21
6.40 ( 0.45
1.83 ( 0.59
0.3 ( 0.3
24 ( 27
0.5 ( 0.5
CASS samples (pH < 5)
10
3.88 ( 0.48
2.06 ( 0.91
16.1 ( 12.4
132 ( 94
3.6 ( 1.7
Results for all groundwater samples are reported in Table S1.
term (i.e., 4 months) daily observations to assess the influence of recurrent floods on groundwater-derived metal exports from the catchment. A total of 31 groundwater samples were collected from shallow wells installed with a hand auger and monitoring wells installed by the Department of Environmental Protection in the 1990s. All wells were constructed with slotted PVC pipes with short screens (usually about 50 cm long) centered at the depths indicated in Table S1. In all cases, we used a peristaltic pump to retrieve samples after purging the wells. The spatial survey in surface waters was conducted in November 2009 about two weeks after a 200 mm rain event. The survey covered the Swamp drains and natural creeks as far upstream as possible. A metal sample was collected every 1 km or when noticeable changes in pH were observed. The time series observations in surface waters captured the drainage of the entire catchment and consisted of daily sampling at low tide in the Tuckean Broadwater immediately downstream of the Tuckean Swamp (Figure 1). Sampling started on 28 January 2010 during dry conditions, captured a flood event, and stopped on 1 June 2010 when radon and pH returned to dry condition values. Analytical Methods. Dissolved element samples were immediately filtered with 0.45 μm disposable acetate filters. The filtered samples were acidified with high-purity HNO3 and stored in acid-cleaned vials. Samples were analyzed for dissolved metals using a Perkin-Elmer DRCe Inductively Coupled Plasma Mass Spectrometer (ICP-MS). Calibrations were performed before and after running the samples and concentrations covered at least 3 orders of magnitude. We accounted for instrument and background drift during analysis by following a standard sample standard bracketing scheme. System blanks were run by treating ultrapure DI water as a sample. The blank concentrations were at least 3 orders of magnitude lower than found in our samples. Water levels, pH, conductivity, and temperature were monitored at 1-h intervals using dataloggers maintained by the Richmond River County Council and calibrated biweekly. Radon (222Rn, t1/2 = 3.84 days) was determined using a continuous, automated radon-in-air monitor adapted for radon-in-water.16 Daily groundwater discharge rates into drains upstream of the surface water monitoring station were derived from a radon mass balance approach as described in detail in a companion paper.17
’ RESULTS AND DISCUSSION Metals in Groundwater. Dissolved metal and pH values in groundwater spanned over 3 orders of magnitude (Table S1). We define groundwater as any water within the saturated zone of geological material18 and group samples as CASS and non-CASS groundwaters (Table 1). The CASS samples were defined as samples with pH < 5, were collected from depths shallower than 2.5 m, and had extremely high metal concentrations and pH as low as 3.11. The non-CASS samples were collected from depths ranging between 0.5 and 33 m and had pH values as high as
Figure 2. Scatter plots between pH and depth and pH and metals in groundwater samples indicating shallow CASS groundwater as the main source of metals to surface waters.
7.61 but usually approaching neutral and metal concentrations 1 order of magnitude lower than the CASS samples. Radon and dissolved oxygen concentrations were very similar in the two groups of groundwater samples. The metals under investigations negatively correlated with pH (n = 31; p < 0.01; r2 > 0.41; Figure 2). These results imply that only shallow CASS groundwaters can be a concomitant source of radon, acid, and dissolved metals to surface waters. If deep non-CASS samples were a major source of groundwater to surface waters, we would not observe significant correlations between radon and trace metals in surface waters (shown later). The extremely high dissolved metal concentrations and low pH in shallow groundwaters from the Tuckean Swamp were consistent with high rates of weathering, iron sulfide mineral oxidation, production of acidity, and an associated release of trace elements from CASS.19 The observed groundwater concentrations were within the wide concentration range typically observed in other (sub-) tropical20 and boreal21 CASS catchments. Metal Distributions within the Tuckean Swamp. A spatial survey (Figure 1) revealed two locations where groundwater was entering surface waters: (1) Stibbards Creek, a natural creek surrounded by nonacidic sandy soils. This creek exhibited high radon concentrations, neutral pH, and relatively low trace metal concentrations. (2) Main Drain, a deepened and straightened drain surrounded by CASS. This area exhibited overall high radon and trace metal concentrations and pH as low as 4.3. The Main Drain captures the discharge from Meerchaum Vale, a groundwater-dominated acidified drain. The metal concentrations 9643
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Figure 3. Time series of trace metals and associate variables in surface waters of the Tuckean Broadwater. Radon data and groundwater fluxes were originally reported in ref 17. Radon and pH values represent 24-h averages, while metal concentrations represent spot samples at low tide.
in this area were within the range observed in similar surveys performed in the 1990s,5,7 demonstrating that trace metal releases from CASS is a long-term problem. Interestingly, two samples collected in Hendersons Drain just upstream of the Main Drain had high Fe but relatively low radon, Mn and Al concentrations, and neutral pH, indicating a surface runoff-derived Fe source in this area. Samples collected from the Stony Island and Tucki Drains had low radon and metal concentrations and neutral pH, implying that these drains are simply a conduit for water from the upper catchment. The survey results demonstrate that not all groundwaters were a source of trace metals to surface waters. Surface water time series observations described below represent an integration of processes taking place within the swamp. Surface Water Time Series. The daily observations captured contrasting hydrological and chemical conditions (Figure 3). Following hydrological/biogeochemical conceptual models for nearby Australian tidal rivers and estuaries,22 we define four stages to describe trace metal distribution:
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1 Dry Period. During the first month of observations, surface waters experienced “slug” flow6 and the net water export during a complete tidal cycle approached zero. Radon concentrations and radon-derived groundwater discharge rates also approached zero. Overall, the water quality was reasonable with high pH and relatively low dissolved metal concentrations. With the onset of the first rain early in February, Fe increased from nearly 0 to 0.5 mg/L, while Al and Mn did not show any change. 2 Flood. A sharp transition from extremely dry to wet conditions was observed in early March. A series of minor rain events in February preceded a strong precipitation (213 mm) event on 2 March. This rain inundated the swamp for approximately one week, increased surface flows to about 120 m3 s 1, increased groundwater flows to a maximum of 5 m3 s 1, and caused a well-defined sequence of chemical changes. Immediately after the rain, during the rising limb of the hydrograph, pH dropped from >6 to about 4.5. During the falling limb of the hydrograph, radon concentrations sharply increased as a result of groundwater inputs into surface waters. While dissolved Al increased slightly during the flood stage to a nearly constant value, Fe and Mn had more distinct changes. Both Fe and Mn peaked at 8.0 and 0.5 mg L 1 during the falling limb of the hydrograph 3 days after the peak surface discharge. The Fe and Mn peak coincided with the radon increase but not with the initial pH decrease. The flood followed a long dry season, which probably allowed weathering products to accumulate in the soil profile. 3 Postflood. The surface water chemistry during the postflood stage was clearly dominated by floodplain groundwater discharge. Radon concentrations remained very high (16 19 dpm L 1) for a week and slowly started to decrease. The radon peak coincided with pH dropping to about 4. The radon decrease toward the end of the postflood stage coincided with a pH increase and an overall decrease in metal concentrations. Fe was 1 order of magnitude lower, Al was 2-fold higher, and Mn was comparable during the postflood stage relative to the flood stage. Radon-derived floodplain groundwater discharge rates reached 2.5 m3 s 1, equivalent to about 15% of the surface runoff. 4 Minor Rains. The time series also captured two minor rain events after the postflood (37 mm on 20 April, and again 37 mm on 4 May 2010). A clear spike in radon and trace metal concentrations followed both rain events. We emphasize that the radon signal detected at the time series station most likely represents groundwater discharge from the CASS floodplain only. While the natural creeks and streams in the upper catchment (Alstonville Plateau) are fed by groundwater, waterfalls and turbulent flow likely degas most of the radon from the upper catchment. The short residence times (about 1 day) and nonturbulent flow in the floodplain drains prevents significant radon degassing within the Tuckean Swamp.11a These observations demonstrate a suspected but poorly understood role played by groundwater and flood events in dissolved metal exports from CASS wetlands. While previous work has not used radon as a groundwater tracer, the sequence of events described for the Tuckean Swamp seems similar to observations made for a CASS catchment in Europe.23 Under dry baseflow conditions, the runoff from areas with CASS was low in comparison to that in non-CASS areas, resulting in small 9644
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Figure 4. Scatter plots between dissolved metals in surface waters and possible controls. The lines on current plots show the hysteresis pattern observed during the flood. The numbers near selected samples are days after the 213 mm rain event that triggered the flood.
loads of trace elements. Rain events following extended dry periods increased both stream discharge and metal concentrations in boreal Finland,23 similar to our observations in warmer Australian conditions. Controls on Surface Water Metal Concentrations. Metal concentrations significantly correlated with radon and pH during the dry period, postflood, and minor rains stage (p < 0.01; Figure 4). While complex biogeochemical and physical processes can influence metal concentrations in surface waters and estuaries, groundwater is the only significant source of radon to surface waters. Other sources are negligible11a and cannot explain the radon concentrations higher than 2 dpm L 1 observed after the rain events in the Tuckean Broadwater. Both shallow and deep groundwaters have high radon concentrations. However, only shallow groundwaters are acidic and high in metals (Table S1). The significant correlations between radon and trace metals (Figure 4) in surface waters imply that their signal was derived mostly (if not all) from shallow groundwater discharge. The shallow soil profile in CASS catchments is often highly permeable due to interconnecting cracks and pores.24 Indeed, hydraulic conductivities in the Tuckean Swamp CASS (10 100 m d 1) are 1 order of magnitude higher than nearby soils,24 allowing groundwater to move quickly, uptake metals from CASS, and discharge to artificial drains. Our observations contrast to work in other systems where surface water discharge was considered the major control on
trace metal concentrations due to a diluting effect.25 In the Tuckean Broadwater, metal concentrations may have been influenced by surface discharges only during the flood. Inspection of scatter plots reveals Fe and Mn counter clockwise hysteresis during the flood associated with a delayed release of metals from the catchment. Fe and Mn concentrations increased slowly during the rising limb of the hydrograph and reached a peak 8 to 10 days after the 213 mm rain event when groundwater discharge was the highest. Metal concentrations started to return to normal values after 11 days. To our knowledge, this is the first observation of dissolved Fe and Mn hysteresis during a flood. Previous studies have measured metal concentrations during rain events in CASS,12,26 but the sampling resolution may have not been enough to detect hysteresis. Total Surface Water versus Groundwater Fluxes. We estimate (1) total surface water metal fluxes at the surface water time series station and (2) groundwater-derived metal fluxes into surface waters upstream of the time series station (Table 2). The total surface water fluxes were estimated by simply multiplying measured dissolved concentrations in surface waters by surface water discharge. The groundwater metal fluxes are a component of the total surface water fluxes. Groundwater fluxes were estimated using the average metal concentrations in groundwater and the daily radon-derived groundwater discharge rates described in a companion paper.17 Large uncertainties may be associated with the groundwater fluxes. Similar to other investigations in coastal groundwaters,27 9645
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Table 2. Average Total Surface Water and Groundwater Fluxes during the Four Different Hydrological Stagesa n (days)
water flux (m 3 s 1)
dry
30
4.35
12
129
22
flood
16
89.24
1567
11164
1029
post flood
27
13.98
951
393
320
minor rains
44
19.73
2868
709
487
dry
30
0.04
12
158
4
flood
16
1.92
809
10315
242
post flood minor rains
27 44
1.19 0.88
499 378
6366 4823
150 113
Al (kg d 1)
Fe (kg d 1)
Mn (kg d 1)
Total Surface Water
Groundwater
a The groundwater fluxes were estimated by multiplying radon-derived groundwater discharge rates by the average metal concentration in all groundwater samples. The surface water fluxes can be converted to aerial export rates using the CASS area in the Tuckean Swamp (4000 ha).
the main issue here is that metal concentrations in groundwaters are highly variable, complicating the definition of endmembers for flux estimates. We take average metal concentrations in all groundwaters as a conservative metal endmember, i.e., these averages provide minimum groundwater fluxes because the source of metals to surface waters is clearly shallow groundwater highly enriched in metals (Table 1). Using average metal concentrations in CASS samples only would triple Al and double Fe and Mn groundwater fluxes. In spite of uncertainties, Al and Mn groundwater fluxes were usually comparable to the total surface water fluxes (Table 2). These observations give confidence in our estimates, support the suggestion of shallow groundwater as the main pathway releasing metals from CASS, and indicate that most of the groundwater-derived Al and Mn were exported to the Richmond River estuary. In contrast to Mn and Al, groundwater-derived Fe fluxes were at least 1 order of magnitude higher than total surface water fluxes during the postflood and minor rains stages. This substantial discrepancy indicates that Fe was sequestered within the drainage system prior to discharge at the time series monitoring station. This observation is consistent with known diagenetic pathways for the formation and accumulation of Fe-rich monosulfidic black ooze (MBO) in the basal sediments of CASS drains.28 As MBO are organic oozes highly enriched in reactive iron mineral phases, their resuspension followed by oxidation during floods could be a source of dissolved Fe enrichment during floods.29 However, the observed delayed Fe input (i.e., counterclockwise hysteresis) implies that groundwater was the major source of Fe. If MBO resuspension was the major source of Fe, the dissolved Fe enrichment should have been observed in the earlier stages of the flood (i.e., clockwise hysteresis). The highest metal concentrations (about 5 mg L 1) observed for 3 days after the surface water discharge peak may be related to an initial wash out of shallow groundwater that had accumulated exchangeable Fe during the dry season. Alternatively, reductive processes in surface waters after the flood could also increase Fe and Mn concentrations. The flood stage lasted for 16 of the 117 days of uninterrupted observations (Table 2). Dissolved Fe, Mn, and Al loads during those 16 days (14% of the time investigated) were respectively 80, 35, and 14% of the total exports from the Tuckean Swamp. These observations underscore the importance of floods in metal exports from coastal catchments and the need to include these events in monitoring programs. Floods can be especially important
in tropical and subtropical estuaries which are often characterized by a higher degree of variability in water flows than the better studied temperate systems.30 Since floods are essentially an “above surface” process, the observed delayed groundwater inputs were previously overlooked during these events. While a number of previous investigations have assessed metal concentrations and loads in surface waters affected by CASS, our link to a groundwater tracer and the high sampling resolution (i.e., daily for 4 months) are unprecedented. Previous work focused on hourly sampling for a few days12 or seasonal sampling for several years.31 For example, a 65-h monitoring of a rain event at a CASS catchment near the Tuckean Swamp demonstrated high and variable trace metal concentrations.12 Previous assessments of groundwater fluxes to coastal waters in acidic environments relied on the assumption that the annual infiltration equals groundwater discharge.21 Our link to high resolution radon measurements revealed exactly when shallow groundwater was flushed from CASS, and that this groundwater was the major source of trace metals to surface waters. A main advantage of using radon as a natural tracer is that radon integrates spatially heterogeneous groundwater pathways that may be difficult to quantify when using conventional hydrological approaches. Global Perspectives. The concentrations of Fe in Tuckean Swamp surface water samples (average ∼1 mg L 1) were orders of magnitude higher than the average metal concentrations in the world’s rivers of
