Identifying sources of dissolved organic carbon in agriculturally dominated rivers using radiocarbon age dating: Sacramento–San Joaquin River Basin, California
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- Sickman, J.O., DiGiorgio, C.L., Lee Davisson, M. et al. Biogeochemistry (2010) 99: 79. doi:10.1007/s10533-009-9391-z
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We used radiocarbon measurements of dissolved organic carbon (DOC) to resolve sources of riverine carbon within agriculturally dominated landscapes in California. During 2003 and 2004, average Δ14C for DOC was −254‰ in agricultural drains in the Sacramento–San Joaquin Delta, −218‰ in the San Joaquin River, −175‰ in the California State Water Project and −152‰ in the Sacramento River. The age of bulk DOC transiting the rivers of California’s Central Valley is the oldest reported for large rivers and suggests wide-spread loss of soil organic matter caused by agriculture and urbanization. Using DAX 8 adsorbent, we isolated and measured 14C concentrations in hydrophobic acid fractions (HPOA); river samples showed evidence of bomb-pulse carbon with average Δ14C of 91 and 76‰ for the San Joaquin and Sacramento Rivers, respectively, with older HPOA, −204‰, observed in agricultural drains. An operationally defined non-HPOA fraction of DOC was observed in the San Joaquin River with seasonally computed Δ14C values of between −275 and −687‰; the source of this aged material was hypothesized to be physically protected organic-matter in high clay-content soils and agrochemicals (i.e., radiocarbon-dead material) applied to farmlands. Mixing models suggest that the Sacramento River contributes about 50% of the DOC load in the California State Water Project, and agricultural drains contribute approximately one-third of the load. In contrast to studies showing stabilization of soil carbon pools within one or two decades following land conversion, sustained loss of soil organic matter, occurring many decades after the initial agricultural-land conversion, was observed in California’s Central Valley.
KeywordsRivers Dissolved organic carbon Radiocarbon Sacramento–San Joaquin Delta XAD resins
Rivers transport the products of rock weathering and terrestrial biogenic materials to the ocean, thereby regulating atmospheric CO2 and O2 concentrations and delivering bioavailable forms of carbon to marine ecosystems (Schlesinger and Melack 1981). Stallard (1998) estimated that total riverine flux of C to the oceans is 0.82 petagrams (Pg) year−1 with the majority of the carbon transported as dissolved and particulate organic matter. The estimated organic matter flux in the World’s rivers, 0.53 Pg C year−1, is approximately equal to two-thirds of annual terrestrial net ecosystem production (NEP; 0.8 Pg C year−1; Xiao et al. 1998).
Increasing concentrations of carbon have been detected in many rivers worldwide and have been attributed to changes in terrestrial carbon balance caused by several factors including: climate change (Freeman et al. 2001: DOC increase; Raymond and Cole 2003: alkalinity increase), CO2 fertilization of terrestrial primary productivity (Freeman et al. 2004: DOC increase), landuse change (Raymond et al. 2008: DIC increase), atmospheric deposition of nitrogen (Findlay 2005: DOC increase) and soil recovery from acid deposition (Monteith et al. 2007: DOC increase). While conversion of natural ecosystems to agricultural production systems is known to reduce soil organic carbon (Paustian et al. 1997; Smith et al. 2001), few studies have specifically assessed how this conversion has impacted DOC loads in large rivers.
Given the multiple contributors of natural organic material to the Delta system, and a general lack of knowledge about soil carbon stability in the Central Valley, a more complete understanding of DOC sources, cycling and fate is needed to understand the impacts of landuse on regional carbon balance, drinking water quality and ecosystem restoration activities. Strong gradients in organic matter age are found in California’s Central Valley; soil carbon may be hundreds to thousands of years old, while carbon recently fixed by algae or crops will have radiocarbon concentrations above the 1950 background level (i.e., bomb-pulse labeled). Thus we hypothesize that radiocarbon provides a means for separating sources of C to the DOC loads in the rivers and estuaries of agriculturally dominated landscapes.
In our study, we sought to answer two questions regarding DOC sources: (1) Does the DOC carried by the two largest rivers of California’s Central Valley derive from destabilization of soil organic matter and (2) what is the dominant source of the DOC load in municipal water exports from Delta? To answer Question 1 we combined measurements of DOC radiocarbon content with other diagnostic characteristics of DOC to qualitatively assess whether soil-derived carbon is major contributor to riverine DOC loads. To answer Question 2 we combined measurements of DOC concentration and radiocarbon content with a hydrologic model of the Delta, to test whether DOC contributions to the California State Water Project can be modeled as a conservative mixture of internal sources (Delta agricultural drainage) and riverine inputs (Sacramento and San Joaquin Rivers). Modeling was conducted using both total DOC and the hydrophobic organic acid fraction (HPOA) which may be primarily derived from humic substances.
Agriculture represents one of the most important drivers of global environmental change particularly with respect to the global carbon cycle. In this manuscript we describe the likely impact of agriculture on the DOC load of two major rivers and their common estuary. Given the wide-spread distribution of agriculturally dominated rivers, particularly in Europe and Asia, our study has relevance to understanding long-term changes in riverine DOC loads caused by global change. Furthermore it confirms the growing utility of radiocarbon as a multi-scalar tracer of the carbon cycle both, qualitatively and quantitatively when combined with mass-flux models to simulate isotope-fluxes (i.e., isoflux). Lastly our study provides critically needed information about DOC sources to the Delta and California State Water Project in the context of ecosystem restoration and drinking water quality.
Surface water sampling and chemical analysis
Surface water samples were collected approximately monthly (April 2003–March 2004), within the Sacramento–San Joaquin Delta, for isolation of DOC for radiocarbon analysis and for other chemical analyses. The Delta sampling sites were: Sacramento River at Hood Station, San Joaquin River at Vernalis, State Water Project at the H.O. Banks Pumping Plant, and agricultural drains on Twitchell, Bouldin and Bacon Islands (Fig. 1). Samples were also collected from small tributaries carrying runoff from the Sierra Nevada in the Kaweah River drainage of Sequoia National Park during April 2005. Since both the Sacramento and San Joaquin Rivers are fed by Sierra Nevada runoff, the tributary samples provided an estimate of boundary conditions for major rivers in the Central Valley of California and were needed in our isotopic mixing models.
We also made radiocarbon measurements on DOC isolates prepared by the U.S. Geological Survey in 2000–2001 with Amberlite XAD 8 and XAD 4 resins and methods of Aiken et al. (1992). A functionally similar resin, Supelite DAX 8 was used for the 2003–2004 Delta samples (see Supplementary Materials). The 2000–2001 samples included the Sacramento and San Joaquin Rivers, the Twitchell Island agricultural drain and the Clifton Court Forebay to the SWP. The 2000–2001 samples were included in our study because many previous investigations have used XAD 8 and XAD 4 to collect DOC and we wanted to verify whether DAX 8, XAD 8 and XAD 4 resins collect DOC of similar radiocarbon content.
The 2003–2004 and 2005 isotope samples were collected in precombusted (3 h at 500°C), 4 l amber glass bottles using a peristaltic pump, Teflon-lined tubing and Pall-Gelman groundwater cartridges (0.45 μm). Procedural blanks were collected on each sampling date and showed no evidence of DOC contamination. At the same time three, 40 ml amber vials (I-Chem VOA) were filled with filtered water for chemical characterization of the DOC. One of the vials was acidified to pH 2 with HCl and the DOC concentration measured on a Shimadzu TOC 5000A analyzer (high temperature combustion: EPA Method 415.1). The detection limit of the analyzer was 0.25 mg l−1 and precision of duplicate samples was better ±15%. Using an unacidified duplicate, we measured absorbance of light at 254 nm in a 1 cm quartz cuvette and divided absorbance by DOC concentration in mg l−1 and multiplied by 100 to yield mass-specific UV absorbance (SUVA) (Standard Methods 5910B). With the last subsample we measured dose-based, mass specific, total trihalomethane formation potential (STHMFP) using a method developed by the California Department of Water Resources (Chow et al. 2006; STHMFP not measured for Sierra Nevada samples).
Preparation of DOC isolates
For samples collected during 2003 and 2004, we isolated and analyzed whole water DOC (WW), and hydrophobic organic acids (HPOA; Leenheer 1981). Whole water DOC was operationally defined as organic carbon that passed through a 0.45 μm Pall-Gelman groundwater filter cartridge and contains both dissolved organic carbon and colloidal organic matter. Aiken et al. (1992) defines the HPOA fraction as organic matter containing aromatic and aliphatic carboxylic acids of five to nine carbons, one- and two-ring phenols, and aquatic humic substances. These compounds supplement microbial foodwebs, attenuate light in the water column and are involved in the complexion of trace metals (Wetzel 2001). Note that we also isolated a transphilic organic acid (TPIA) fraction using XAD-4 resin, but recoveries were less than 10–15% of the total DOC loaded. The TPIA fraction is believed to be functionally similar to the HPOA fraction and contains organic acids with fewer than five C atoms. Because our isolation approach used relatively small collection volumes (<5 l), the high ratio of inorganic salt to organic matter in the TPIA isolates prevented reliable Δ14C analysis and quantitative estimates of TPIA mass, so no data are reported.
To prepare WW isolates, we acidified the water samples to pH 2 with trace metal grade HCl and used rotary evaporation alone (in vacuo at 40°C) or rotary evaporation combined with lyophilization to produce a powder. For HPOA isolates, a duplicate filtered and acidified sample was loaded onto a column filled with the polymeric adsorbent, DAX 8 (lot numbers XAD-8-201 and XAD-8-4004; Supelco, Bellefonte, PA) and eluted with 0.1 N NaOH following the procedures of Aiken et al. (1992) (see Supplementary Materials). The eluates were acidified to pH 2 with HCl and evaporated as above to a powder.
All WW and HPOA isolates were analyzed for radiocarbon content. Samples were combusted in evacuated quartz tubes with CuO and the resultant CO2 was purified and isolated cryogenically on a vacuum extraction line. All glassware and reagents were combusted at 500°C prior to use. The CO2 samples were then reduced to graphite at high temperature in the presence of H2 and formed into graphite targets for 14C measurement by accelerator mass spectrometry (Vogel et al. 1987). Graphitization and radiocarbon analysis were performed at the Center for Accelerator Mass Spectrometry at Lawrence Livermore National Laboratory. Radiocarbon concentrations are reported in Δ14C units relative to the NIST Oxalic Acid I standard and corrected to a δ13C value of −25‰ (Stuiver and Polach 1977).
The laboratories where water samples were processed underwent regular swipe tests to monitor background 14C levels and potassium hydrogen phthalate blanks were run routinely and showed no evidence for significant 14C contamination (mean Δ14C value = −996 ± 0.5‰; n = 10). Under basic conditions, the DAX 8 resin bled a small amount of organic carbon and a procedure was developed to correct for the effect of the bleed on the radiocarbon analyses (see Supplementary Materials).
Summary of expected errors in WW Δ14C, HPOA Δ14C and non-HPOA Δ14C including individual component errors
1. Mass: WW DOC
Rivers and SWP
2. Mass: HPOA DOC
Rivers and SWP
3. Mass: non-HPOA DOC
Rivers and SWP
4. Resin bleed
Rivers and SWP
5. Δ14C: whole water
Rivers and SWP
6. Δ14C: HPOA
Rivers and SWP
7. Δ14C: non-HPOA
Rivers and SWP
The WW and HPOA isolates from April to August 2003 were also analyzed for δ13C. Low variability in the δ13C of the samples led us to abandon these measurements after August 2003. Stable isotope analyses were performed on a Delta-Plus XP isotope ratio mass spectrometer using an elemental analyzer inlet system and continuous flow of He. Standard sucrose from the International Atomic Energy Association was included in each run. Isotope abundances are reported in δ notation relative to the standard PeeDee Belemnite.
Hydrologic and isotopic mixing models
To compliment the DSM2 mixing model we employed an alternative isotope-mixing model known as Isosource. Isosource (Phillips and Gregg 2003) was used to determines DOC source contributions based on stable isotope and chemical composition when sources of unknown mass were considered (e.g., DOC supplied by primary producers). Under these conditions a unique mixing solution cannot be reached so, instead, Isosource calculates ranges of possible contributions to a mixture to satisfy isotopic mass balance in the mixing model.
Chemical and isotopic composition of DOC fractions
For presentation, monthly chemical and isotopic characteristics of DOC (see Auxiliary Materials, Table 1) were averaged for the following defined seasons: spring (April–June), summer (July–September), fall (October–December) and winter (January–March). Aggregation of the data also allowed for statistical testing of differences among sites and was more appropriate for examining intra-annual patterns given the approximately 28 day residence time of the Delta (Jassby and Cloern 2000).
For the 2003–2004 samples, there were no significant differences in trihalomethane formation potential per mole (STHMFP) among the sample types for the overall data set, although significant differences were noted among samples types for specific seasons, e.g., drains had higher STHMFP than the Sacramento River during the Spring (Fig. 2c). Mean drain values ranged from 9.8 to 11.7 mmol THM per mole DOC and mean river and SWP values ranged from 8.6 to 12.1 mmol THM per mole DOC.
The radiocarbon content of the 2003–2004 HPOA fractions generally declined moving from spring into the summer and into the fall (Fig. 3b). ANOVA performed for the entire dataset indicated significantly lower HPOA Δ14C values in the agricultural samples relative to both rivers (p < 0.001), however there was no statistical differences between the HPOA values in the agricultural drains and the SWP. Seasonal HPOA Δ14C ranged from −150 to −265‰ in agricultural drains, +195 to +21‰ in the Sacramento River, +245 to −19‰ in the San Joaquin River and +118 to −122‰ in the SWP (Fig. 3b).
The stable carbon isotope composition of DOC varied less than 3‰ both among sites and among the DOC fractions for a single site (Fig. 3c). Average (±SE) δ13C of WW DOC were −26.6 ± 0.2, −26.5 ± 0.7, −26.0 ± 0.2 and −26.8 ± 0.1‰ for agricultural drains, the Sacramento River, the San Joaquin River and the SWP, respectively.
Additional radiocarbon measurements of dissolved organic carbon made in the Central Valley of California and in the Kaweah River basin of the Sierra Nevada
Raw HPOA Δ14C ‰
Raw TPIA Δ14C ‰
Twitchell Island (Agricultural drain)
Sacramento River (Hood Station)
San Joaquin River (Mandeville Island)
San Joaquin River (Prisoners Point)
San Joaquin River (Vernalis Station)
State Water Project (Clifton Court Forebay)
Kaweah River Basin
Marble Fork of Kaweah (Subalpine zone)
Wall Spring—Giant Forest (Mixed conifer zone)
Elk Creek (Chaparral zone)
Horse Creek (Foothill grassland zone)
Samples of WW DOC were collected during spring snowmelt in the Kaweah River drainage within Sequoia National Park in order to characterize the radiocarbon content of organic matter produced from undisturbed landscapes (Table 2). With the exception of Wall Spring, WW-Δ14C was less than modern and ranged from −58 to −171‰.
The mass and radiocarbon content of the WW and HPOA fraction were used with Eq. 1 to estimate the non-HPOA Δ14C values by sample type for each season. For all of the 2003–2004 samples at each site, the non-HPOA fraction had significantly lower Δ14C values (p < 0.001) than either the WW or HPOA fractions. Seasonal non-HPOA Δ14C values ranged from −282 to −354‰ in agricultural drains with an uncertainty of ±25%. Seasonal non-HPOA Δ14C ranged from, −230 to −437‰ in the Sacramento River, −275 to −687‰ in the San Joaquin River and −258 to −479‰ in the SWP. In the river and SWP samples, computation of non-HPOA Δ14C was complicated by low DOC concentrations and greater influence of resin bleed thus uncertainty of non-HPOA Δ14C measurements is ±37%. Based on ANOVA run for the entire data set, there were no significant differences in non-HPOA Δ14C among the four samples types or within any season.
Relationships among isotopic and chemical characteristics of DOC
There were statistically significant correlations among the Δ14C of WW, HPOA and TPIA DOC (corrected and uncorrected for resin bleed) for individual Delta samples collected during 2003–2004 and 2000–2001 (Fig. 4a, b). For the bleed-corrected and WW samples collected during 2003–2004, the Pearson r was 0.47 (p < 0.001); for samples collected with XAD 8 resin during 2000–2001, the Pearson r between the HPOA and WW fractions was 0.69 (p = 0.026). Wilcoxon signed rank tests demonstrated that WW-Δ14C was significantly lower than bleed-corrected HPOA Δ14C for the 2003–2004 (p < 0.001) and 2000–2001 samples (p = 0.059). There was a strong linear correlation (r2 = 0.99) between the Δ14C of HPOA and TPIA DOC in the 2000–2001 samples and paired t-testing indicated no significant difference (p = 0.81) between the Δ14C of these DOC fractions.
No significant relationships were detected between the Δ14C or δ13C (not shown) of WW DOC and measures of the mass-specific UV absorbance and disinfection byproduct formation potential in individual samples when agricultural samples were separated from riverine and SWP samples (Fig. 4c). However, when all samples were combined there was a significant, negative correlation between SUVA and WW-Δ14C, i.e., older DOC had greater aromaticity (Pearson r = −0.32; p = 0.009). Interestingly, despite a range of more than 400‰ in Δ14C of WW DOC (corresponding to an age range of >3700 radiocarbon years), formation potential was predominantly between 9 and 12 mmol THM per mole C.
Hydrologic and isotope mixing models
The fractional contributions of water for each source were averaged for individual months and then combined with DOC, HPOA and radiocarbon concentrations (WW and HPOA), and SUVA and STHMFP using Eqs. 2 and 3 to test the hypothesis that DOC concentration and chemical/isotopic composition at the SWP intake can be predicted from conservative mixing of water sources (Fig. 5b–d). The model produced good agreement with measured DOC concentration and trends, with an adjusted r2 value of 0.91. However, the slope of the regression line, 1.23 (x = modeled, y = measured for all regressions), was significantly different from 1 and indicated the model under-predicted DOC concentrations at the SWP intake (Fig. 5b).
Modeled WW-Δ14C closely matched measured Δ14C in only three of twelve months (adjusted r2 = 0.32), however, the model did capture upward and downward trends in WW-Δ14C values (Fig. 5b). There was strong bias in the model during two periods: June through September when measured WW-Δ14C was higher (younger) than predicted and December through February when measured WW-Δ14C was lower (older) than predicted. For the months of August and December, we used interpolated values for WW-Δ14C in the San Joaquin River, since these samples were lost. However, since the fractional contribution of water from the San Joaquin River for these months was 2.8 and 7.8%, respectively, we believe that interpolation had little effect on the modeling results.
The mixing model for HPOA concentration and HPOA Δ14C were successful in predicting conditions at the SWP intake (Fig. 5c). Differences between measured and modeled HPOA were typically less than 0.5 mg C l−1 in all months and exhibited no bias; the adjusted r2 value was 0.83 and the slope of the regression was 1.06 and not significantly different from 1. Modeled HPOA-Δ14C closely followed month-to-month changes in HPOA-Δ14C at the SWP intake (Fig. 5c). The only periods when significant biases were observed were: (a) August and September where modeled HPOA-Δ14C was lower than measured HPOA-Δ14C and (b) December and January where modeled HPOA-Δ14C was higher than measured values. These biases in HPOA were consistent with WW-Δ14C biases. The adjusted r2 between measured and modeled HPOA-Δ14C was 0.83 and the slope of the regression 0.96, was not significantly different from.
In contrast to DOC and radiocarbon concentrations, mixing models incorporating mass-specific UV absorbance and THMFP performed poorly. The models generally failed to capture the month-to-month patterns observed at the SWP intake (Fig. 5d) and regressions between modeled and measured SUVA and STHMFP yielded non-significant slopes and low r2 values. The models also produced several periods of substantial bias for both parameters.
Sources of riverine DOC in agriculturally dominated rivers
Radiocarbon measurements of DOC in streams and rivers world-wide
Whole-water Δ14C ‰
HPOA Δ14C ‰
Mayorga et al. (2005)
94 ± 176
196 ± 59
177 ± 64
Raymond et al. (2007)
78 ± 25
14 ± 51
62 ± 47
5 ± 53
7 ± 35
Striegl et al. (2007)
Evans et al. (2007)
37 ± 5
66 ± 10
−63 ± 124
−53 ± 30
38 ± 31
(*Low Flow Periods only)
22 ± 20
Baffin Island, Canada
Abbott and Stafford (1996)
60 ± 59
Eastern United States
89 ± 77
210 ± 40
82 ± 48
145 ± 149
Stern et al. (2007)
Stormwater Treatment Area
−198 ± 28
San Joaquin River
−218 ± 99
91 ± 159
−152 ± 103
76 ± 139
−254 ± 32
−204 ± 52
State Water Project
−175 ± 90
2 ± 142
Comparison of organic carbon pools and riverine loads in the Central Valley of California
Organic C pool/load
Soil organic carbon in top meter of soila
Annual loss of soil organic carbon
Annual DOC load in the Sacramento and San Joaquin Riversb
Non-HPOA load in the Sacramento and San Joaquin Riversc
Agrochemical application (active ingredient)d
The average Δ14C of the HPOA fraction (+76 to +91‰) in the Sacramento and San Joaquin rivers are similar to those reported in other rivers and lakes (e.g., Hedges et al. 1986; Abbott and Stafford 1996; Leenheer et al. 2004). More importantly, HPOA-Δ14C values were higher than the WW-Δ14C and, consequently, the Δ14C of the non-HPOA computed from Eq. 1 consistently yielded significantly lower non-HPOA Δ14C values, especially for the San Joaquin River where seasonal averages ranged from −275 to −687‰. Most samples of the Sacramento and San Joaquin Rivers had WW and non-HPOA Δ14C values significantly lower than WW-Δ14C measured in undisturbed areas of the upper Sierra Nevada watershed, indicating older DOC is added to rivers as they pass through regions of intensive agricultural production.
Low WW-Δ14C values are ubiquitous in the Sacramento and San Joaquin rivers and in general appear to be related to disturbance of soils by agricultural activities and urbanization (Sickman et al. 2007). In contrast, it is not straightforward to conclude a soil source of especially old, non-HPOA. Ewing et al. (2006) investigated the effects of soil structure and soil disruption on the Δ14C of soil organic carbon and concluded that soil structures (peds) in San Joaquin Valley soils offer physical protection to soil organic carbon over millennial time scales even when the organic matter is not biochemically recalcitrant. During incubation studies of intact grassland and orchard soils with high clay content, Ewing et al. found that disruption of the peds produced CO2 that was considerably more depleted in 14C than the bulk soil; in one experiment, the Δ14C of CO2 produced from B horizon orchard soils ranged between −716 to −908‰. Implicit in the biodegradation of old soil carbon is the incorporation of aged carbon in microbial cell mass. In this case, the growth and decay cycles of the microbial mass could contribute to the DOC pool as well, adding saccharide and amino compounds from lyzed cells that have an apparently old age. Evidence for abundance of these compounds in colloidal hydrophilic fractions in terrestrial and marine water DOC is increasing (Guo and Santschi 1997; Leenheer et al. 2004; Kawasaki and Benner 2006; Jorgensen et al. 2008). This source of organic matter would be consistent with the hydrophilic fraction of DOC in the Sacramento and San Joaquin rivers, and its apparently old age a result of the intense cultivation and subsequent biodegradation of previously occluded but now exposed soil organic matter. This source of old hydrophilic DOC would also explain the distinct apparent age difference between the HPOA and the non-HPOA fractions.
The aged non-HPOA measured in our study may also be 14C-depleted materials derived from petroleum; examples include agrochemical active ingredient and oil-based carriers and/or their decomposition products. Other sources of fossil carbon include oils in non-point urban runoff, treated sewage containing synthetic detergents and polyacrylamide which is commonly used to control soil erosion both on farms and construction sites. Interestingly, black carbon from fossil fuel combustion has also been detected in large rivers (Mitra et al. 2002) and evidence is growing that BC is an important component of the global carbon cycle (Mannino and Harvey 2004).
The possibility that fossil carbon might contribute to DOC loads in rivers was first proposed by Spiker and Rubin (1975). Using a simple isotope mixing model, they estimated that fossil carbon contributed as much as 20–30% of the total DOC load in several rivers along the U.S. east coast. However, their mixing model did not take into account that DOC derived from soils could be hundreds to thousands of years old, thus their model overestimated the true contribution of oil-based chemicals in these rivers.
Estimating the mass of agrochemical carbon applied to land in the Central Valley is complicated by the large number of compounds used and lack of data on petroleum carriers employed during the field application process. Based on data collected from the California Department of Pesticide Regulation and summarized by the Pesticide Action Network (Kegley et al. 2008), we estimate that 0.087 teragrams of carbon are applied as pesticide active ingredient in the Central Valley and that total carbon applications are likely several times greater (Table 4). Agrochemical carbon application roughly equals the total DOC load in the Sacramento and San Joaquin Rivers, however more research is needed to understand the chemical and physical transformations necessary to solubilize and transport these compounds to rivers.
Trends in DOC loads in agriculturally dominated rivers
Trends in the carbon loads of major rivers are indicators of changes in regional carbon balance caused by global environmental change. Observed ground subsidence within the Sacramento–San Joaquin Delta (Mount and Twiss 2005) demonstrates widespread loss of soil organic matter, but investigations of water quality trends in the Central Valley provide no evidence for increasing riverine DOC concentrations between 1980 and 2000 (Saleh et al. 2003). We speculate, however, that a longer of record of DOC concentrations extending to the early twentieth century would show increases in the San Joaquin and Sacramento Rivers given results from our radiocarbon study and data on radiocarbon dating of DOC in other rivers.
Worrall and Burt (2007) reported that DOC concentrations increased at 216 out of 315 river monitoring stations in Great Britain between 1975 and 2002 and radiocarbon analysis of DOC in these rivers point to complex source mechanisms for increasing DOC concentrations. Evans et al. (2007) found that rivers in the United Kingdom (UK), carrying runoff from undisturbed peatlands, had DOC composed primarily of carbon fixed during the past 50 years. This finding suggests that DOC in the Delta and Central Valley rivers, prior to agricultural reclamation, was much younger than today. Radiocarbon measurements made by Evans et al., in rivers draining intensively managed farmlands had average Δ14C of ca. −60‰ (~500 radiocarbon years) indicating soil carbon destabilization caused by agriculture, although the degree of soil organic matter loss is more modest than that occurring in the Central Valley of California. Further radiocarbon measurements by Billett et al. (2007) in other UK peatlands revealed that, while the riverine DOC pool consisted primarily of bomb-labeled C, the DIC pool was derived from a more 14C-depleted source (older soil organic matter or carbonate minerals) with mean age of 500 to 1,500 radiocarbon years (Δ14C-DIC was not measured in our study). While agricultural practices and perhaps urbanization explain increasing trends in DOC concentration from disturbed landscapes in the UK, some other mechanism, such as decreased sulfur deposition and recovery of soils from acid rain, may be causing destabilization of soil carbon pools in non-agricultural UK lands.
DOC loading to the State Water Project
Recent studies using both mass balance (Jassby and Cloern 2000; Tetra Tech Inc. 2006) and DOC fingerprinting (Kraus et al. 2008) techniques have found that riverine inputs are the dominant source of DOC to the Delta on an annual basis. Our mixing models based on isotopic and chemical fingerprints of DOC and HPOA (Eq. 3) show that organic matter may undergo substantial chemical transformations (as evidenced by poor modeling of SUVA and STHMFP) and the overall organic matter pool maybe turning over (as evidenced by biases in WW and HPOA-Δ14C) as river water moves through the Delta. This finding increases the uncertainty in estimates of monthly DOC and HPOA contributions to the SWP intake, particularly in periods of the year when the fingerprinting models show substantial bias (e.g., summer and winter months in Fig. 5b, c). Nevertheless, source contributions may be estimated with reasonable accuracy on an annual basis if one assumes that: (a) poor SUVA and STHMFP model performance arose from chemical changes rather than unaccounted for inputs or losses of DOC, (b) errors arising from turnover of the DOC pool compensate over the course of a year and (c) DSM2 volume fractions are accurate. With these assumptions, our mixing models suggest that 33% of the annual WW and HPOA DOC load in the SWP is contributed by Delta agricultural drainage and 66% is contributed by riverine input. By itself we estimate that the Sacramento River contributes 50% of the annual WW and HPOA DOC load in the SWP. Given, the relatively narrow range of STHMFP values measured in our study, DBP precursor source proportions should be similar (Fig. 4c).
The WW-Δ14C model consistently predicted older carbon than was observed at the SWP intake during the late spring and summer, and it predicted younger carbon than was observed at the SWP intake during December through February when river discharge was highest. In this latter case, we hypothesize that the DSM2 model underestimated winter-time pumping from agricultural drains resulting in underestimation of WW DOC concentration and overestimation of WW- and HPOA-Δ14C (Fig. 5b). In contrast, since the DSM2 model produced relatively good fits between predicted and modeled WW and HPOA DOC concentrations for the growing season, we hypothesize that the spring/summer bias in our WW-Δ14C models derived from replacement of older riverine DOC with younger DOC from primary production (i.e., turnover of the DOC pool) during the summer. This hypothesis is supported by mass balance studies which show increasing contributions of wetland DOC during the spring and summer growing season (Fleck et al. 2007). Moreover, the average WW-Δ14C at the SWP during the summer fell above (i.e., was younger than) the range of WW-Δ14C for any of the measured sources (Fig. 3a), thus there must have been a younger source of Delta DOC contributing to SWP DOC loads. Using the alternative mixing model, Isosource and three potential end members (average summer WW-Δ14C values for agricultural drains (−255‰), and riverine inputs (−237‰), along with primary productivity with Δ14C value of 50‰) we estimate that 17–35% of the WW DOC at the SWP intake in the months of July–September 2003 was recently fixed organic matter. Kraus et al. (2008), using an independent DOC fingerprinting method, predicted that 30% and 16% of DOC in Clifton Court Forebay during May and July 2000 was contributed by recent primary production in Delta wetlands.
In contrast to earlier studies that examine carbon loss from ecosystems following conversion to agriculture, ours is one of the first to suggest that old carbon losses continue for decades after the conversion process is complete. Mean radiocarbon values for WW DOC in agricultural drainage, the Sacramento River, the San Joaquin River and State Water Project were: −254 ± 99, −152 ± 103, −218 ± 99 and −175 ± 90‰, respectively. Hydrophobic acid (HPOA) fractions in the riverine samples were significantly younger and averaged, −204 ± 52, 76 ± 139, 91 ± 159 and 2 ± 142‰ for agricultural drains, Sacramento and San Joaquin River and the intake of the State Water Project, respectively. The WW DOC radiocarbon values measured in our study were older than the HPOA values; this finding held true for agricultural drain samples, and both of the major rivers in the Central Valley of California. Using mass balance, we estimated the concentration and radiocarbon content of the non-HPOA DOC fraction which fell below −500‰ in some riverine samples. The source of this extremely old DOC is unknown, but we speculate that it derives from physically protected soil organic matter that is being disturbed by human activities in the Central Valley or petroleum-based compounds. Mixing modeling using radiocarbon and other chemical characteristics of DOC suggest that the Sacramento River is the single largest source of DOC to the State Water Project. Primary productivity contributes 17–35% of the DOC at the SWP intake during the growing season and loading from agricultural drain contributes about one-third of annual DOC load in the State Water Project.
This study was a collaborative effort of Municipal Water Quality Investigations of the Department of Water Resources (DWR). We specifically thank DWR Field Staff, David Gonzalez, Steven San Julian, Walt Lambert and Arin Conner for assistance with water sampling. We thank the Center For Accelerator Mass Spectrometry at Lawrence Livermore National Laboratory for conducting the radiocarbon measurements. Supporting laboratory analyses were conducted by the Bryte Analytical Laboratory. Funding was provided by the CALFED Bay Delta Program and the California State Water Contractors.
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