Role of metal species in flocculation rate during estuarine mixing

Flocculation can be considered as an effective mechanism in self-purification of metals during estuarine mixing. In the present investigation, flocculation of metals during mixing of Minab River water with the Strait of Hormuz (The Persian Gulf) water is studied for the first time. Flocculation behavior of metals (except for Pb) is governed by dissolved organic carbon. The source of dissolved organic carbon is terrigenous in the estuarine waters of study area. The general pattern of flocculation of studied metals is manganese (180 μg/L) > zinc (88 μg/L)> nickle (73 μg/L)> copper (30 μg/L)> lead (19 μg/L). The results of present study show that metal species are a very important factor in overall flocculation rate. It is found that solids and oxides have the highest and lowest flocculation levels, respectively. Eh-pH diagram indicated that lead is present as lead oxide in Minab River water and the least flocculation rate is attributed to this element. The results also showed that flocculation rate of metal species could be as solids > free ions ≈ hydroxides > oxides. The amount of metal flocculation is about 30.5, 6.6, 25.3, 10.4 and 62.5 ton/y for zinc, Pb, Ni, Cu and Mn, respectively.     

Migration of dissolved trace elements in the mixing zone between Volga River water and Caspian seawater: Results of observations over many years

Based on natural observations over many years, the distribution of dissolved nutrients and trace elements was analyzed in the mixing zone between the freshwater of the Volga River and Caspian seawater. Most of the trace elements (Li, Rb, Cs, B, F, Br, I, Ga, Sc, Y, Co, Ni, Cu, Cd, Ag, V, As, Sb, Bi, Mo, W, and U) show a conservative behavior. During the period of the highest bioproductivity, dissolved phosphates and silica are intensely removed from solution (up to 60–90 and 46–82% of their supply by river runoff, respectively) mostly owing to uptake by aquatic organisms. The distribution of dissolved strontium was assigned to the weakly nonconservative type, because a minor excess of its content above the lines of conservative mixing (8–18%) was observed in some years; perhaps, this is related to different water transformation at the areas of moving and stagnant water in the delta and offshore mouth zone. Barium is characterized by additional input into the solution (up to 52%) in the regions of medium salinity owing to ion exchange reactions in the absorbed complex of river suspended material. The migration of dissolved species of aluminum, manganese, and iron in the mixing zone of Volga and Caspian waters is probably controlled by the coagulation and flocculation of organic and organo-mineral colloids, which is indicated by a sharp decrease in the content of these elements during the initial stage of salinization (59, 91, and 74%, respectively) followed by a plateau. The most complicated distribution was observed for titanium, lead, and rare earth elements (REE), the concentrations of which showed intense removal from the solution (up to 64–88% Ti, 52–87% Pb, and 66–83% REE) followed by a gradual increase, which is probably related to the elevated contents of these elements in the water of the northern Caspian above a local minimum in the zone of active flocculation of colloids.     

Metals removal during estuarine mixing of Arvand River water with the Persian Gulf water

In the present study, the removal of dissolved and colloidal Cd, Co, Cu, Ni and Zn in Arvand River water during estuarine mixing with the Persian Gulf water is investigated. The flocculation process was investigated for a series of mixtures with salinities ranging from 0.48 to 30.3^. The flocculation rates were indicative of the non-conservative behavior of studied metals during estuarine mixing. Rapid flocculation in the low salinity regimes was observed. The order of the final flocculation rate of metals in the river water was as follows: Co (91.2%)> Cd (86.9%)> Zn (83%)> Cu (75.2%)> Ni (74.3%). Salinity, pH, EC and dissolved oxygen do not govern the flocculation of metals during estuarine mixing. The results of the present investigation show that estuarine processes can be considered as an effective mechanism in self purification of colloidal metals that are anthropogenically introduced into the fresh water ecosystem.     

Flocculation of heavy metals during mixing of freshwater with Caspian Sea water

Flocculation of colloidal size fraction for Cu, Zn, Pb, Ni and Mn was investigated on a series of mixtures with water salinities ranging from 1.5 to 9.5‰ during mixing of Haraz River with the Caspian Sea water. The flocculation trend of Zn (85.5%) > Mn (55.2%) > Cu (39.6%) > Pb (33.7%) > Ni (11.3%) indicates that Cu, Zn, Pb and Mn have non-conservative behavior and Ni has relatively conservative behavior during estuarine mixing. Highest flocculation of heavy metals occurs between salinities of 1.5 and 4.5‰. Statistical analysis indicates that the flocculation of Cu, Zn and Ni is governed by pH and total nitrogen.     

Iron isotope systematics in estuaries: The case of North River, Massachusetts (USA)

Recent studies have suggested that rivers may present an isotopically light Fe source to the oceans. Since the input of dissolved iron from river water is generally controlled by flocculation processes that occur during estuarine mixing, it is important to investigate potential fractionation of Fe-isotopes during this process. In this study, we investigate the influence of the flocculation of Fe-rich colloids on the iron isotope composition of pristine estuarine waters and suspended particles. The samples were collected along a salinity gradient from the fresh water to the ocean in the North River estuary (MA, USA). Estuarine samples were filtered at 0.22 μm and the iron isotope composition of the two fractions (dissolved and particles) were analyzed using high-resolution MC-ICP-MS after chemical purification. Dissolved iron results show positive δ⁵⁶Fe values (with an average of 0.43 ± 0.04‰) relative to the IRMM-14 standard and do not display any relationships with salinity or with percentage of colloid flocculation. The iron isotopic composition of the particles suspended in fresh water is characterized by more negative δ⁵⁶Fe values than for dissolved Fe and correlate with the percentage of Fe flocculation. Particulate δ⁵⁶Fe values vary from −0.09‰ at no flocculation to ∼0.1‰ at the flocculation maximum, which reflect mixing effects between river-borne particles, lithogenic particles derived from coastal seawaters and newly precipitated colloids. Since the process of flocculation produces minimal Fe-isotope fractionation in the dissolved Fe pool, we suggest that the pristine iron isotope composition of fresh water is preserved during estuarine mixing and that the value of the global riverine source into the ocean can be identified from the fresh water values. However, this study also suggests that δ⁵⁶Fe composition of rivers can also be characterized by more positive δ⁵⁶Fe values (up to 0.3‰) relative to the crust than previously reported. In order to improve our current understanding of the oceanic iron isotope cycling, further work is now required to determine the processes controlling the fractionation of Fe-isotopes during continental run-off.     

Effect of pH and salinity on flocculation process of heavy metals during mixing of Aras River water with Caspian Sea water

The flocculation process of metals can play an effective and important role in self-purification of metals during the mixing of freshwater with seawater in estuary. Such processes are of highly ecological and biological importance. The present study deals with the effect of pH and salinity on the flocculation process of dissolved Cu, Mn, Ni, Zn and Pb on a series of mixtures with salinities ranging from 0.5 to 2.5 ‰ with various pHs values (pH 7, 7.5 and 8) during the mixing of the Aras River water with the Caspian Sea water. The flocculation trend of Pb (100 %) > Ni (62.5 %) > Zn (30.43 %) > Mn (25 %) > Cu (18.18 %) at different salinity regimes (0.5–2.5 ‰) at pH 7, indicates well that Pb, Ni, Zn and Mn have non-conservative behavior and Cu has relatively conservative behavior. At various salinity ranges (0.5–2.5 ‰) and pH 7.5, the flocculation trend of Pb (100 %) > Ni (62.5 %) > Mn (37.5 %) > Cu (24.24 %) > Zn (17.39 %) indicates that Pb, Ni, Mn and Cu have non-conservative behavior and Zn has relatively conservative behavior. Also, the flocculation trend of Pb (100 %) > Zn (78.26 %) > Ni (62.5 %) > Mn (37.5 %) > Cu (15.15 %) at different salinities (0.5–2.5 ‰) and pH 8, indicates that Pb, Zn, Ni and Mn have non-conservative behavior and Cu has relatively conservative behavior. Cluster analysis indicates Mn and Ni are mainly governed by salinity. According to the mean annual discharge of the Aras River (5,323 × 10⁶ m³/year), the annual discharge of dissolved Cu, Mn, Ni, Zn and Pb into the Caspian Sea would reduce from 175.66, 85.17, 85.17, 1,224.29 and 53.23 to 149.04, 53.23, 31.94, 266.15 and 0.00 ton/year, respectively.     

Origin and chemical partitioning of heavy metals in riverbed sediments

In the present investigation, bulk and chemical partitioning of elements in the Shefa-Rud riverbed sediments are studies. Higher concentrations of elemental concentrations have been observed in estuarine zone when compared with riverine sediments (except for AI, Fe, Pb and Mn). Manganese is mobilized under anoxic conditions prevailing in the Caspian Sea. Lithogenous materials are greatly diluted in the estuarine zone by various pollutants present in the Caspian Sea. Organic metallic bonds are not significantly present in the area of study. Geological units of the area of study have resulted in the lower concentrations of elemental concentrations of riverbed sediments when compared with published values for mean crust and world sediments ones. Though, cluster analysis has clearly shown the importance of alumina-silicates in controlling the distribution of Fe and Mn in riverbed sediments but it could not depict controlling mechanism for other studied elements. Geochemical Index (Igeo) and Enrichment Factor (EF) values are indicative of a clean environment throughout the river course. These values are in a well agreement with results of chemical partitioning data. Quantification of EF values is not logically possible and therefore Igeo values can be used more effectively.     

Chemical transformation of the runoff of dissolved matters in the mouth areas of the Onega and Mezen’ rivers

Natural observations were analyzed to study the distribution of dissolved species of major and trace elements in the Onega and Mezen’ mouth areas and the tendencies in the chemical transformations of the is continental runoff in the river mouths of the White Sea drainage system. It is shown that the migration of major ions and dissolved species of Li, Rb, Cs, Sr, B, F and Mo is consistent with a conservative behavior and is controlled by hydrodynamic processes. The amounts of uranium and barium additionally supplying in the Mezen’ mouth exceed those removed with a continental runoff, whereas the Onega, Severnaya Dvina, and other rivers of the White Sea drainage system are characterized by the conservative behavior of uranium, while barium desorption from particulate matter reaches no more than 33% of its content in the riverine waters. The growth of concentrations of these elements in the Mezen’ mouth is caused by the long-term interaction of solid matters of the continental runoff with saline waters in the tide-affected estuary. 28–59, 12–63, 25–67 and 20–63% of concentrations of iron, aluminum, lanthanum, and cerium are removed from the riverine waters in the mouth areas of all studied rivers of the White Sea drainage system mainly owing to the coagulation and flocculation of organic and organomineral colloids. The distribution of dissolved species of mineral phosphorus and silicon in the Mezen’ mouth is presumably controlled by the remineralization of the organic matter in the bottom sediments, which due to the hydrological features of estuary are regularly stirred up and interact with vertically mixing water sequence. Up to 20–46% of dissolved phosphates and 3–22% of silicon are removed from the continental runoff during vegetation period in the mouths of the Onega, Severnaya Dvina, and other rivers of the White Sea drainage system mainly owing to their biological consumption.     

Increased concentrations of dissolved trace metals and organic carbon during snowmelt in rivers of the alaskan arctic

Arctic rivers typically transport more than half of their annual amounts of water and suspended sediments during spring floods. In this study, the Sagavanirktok, Kuparuk and Colville rivers in the Alaskan Arctic were sampled during the spring floods of 2001 to determine levels of total suspended solids (TSS) and dissolved and particulate metals and organic carbon. Concentrations of dissolved organic carbon (DOC) increased from 167 to 742 μmol/L during peak discharge in the Sagavanirktok River, at about the same time that river flow increased to maximum levels. Concentrations of dissolved Cu, Pb, Zn and Fe in the Sagavanirktok River followed trends observed for DOC with 3- to 25-fold higher levels at peak flow than during off-peak discharge. Similar patterns were found for the Kuparuk and Colville rivers, where average concentrations of dissolved trace metals and DOC were even higher. These observations are linked to a large pulse of DOC and dissolved metals incorporated into snowmelt from thawing ponds and upper soil layers. In contrast with Cu, Fe, Pb and Zn, concentrations of dissolved Ba did not increase in response to increased discharge of water, TSS and DOC. Concentrations of particulate Cu, Fe, Pb and Zn were more uniform than observed for their respective dissolved species and correlated well with the Al content of the suspended particles. However, concentrations of particulate Al were poorly correlated with particulate organic carbon. Results from this study show that >80% of the suspended sediment and more than one-third of the annual inputs of dissolved Cu, Fe, Pb, Zn and DOC were carried to the coastal Beaufort Sea in 3 and 12 d, respectively, by the Kuparuk and Sagavanirktok rivers.     

The mechanism of iron removal in estuaries

A survey of U.S. east coast estuaries confirms that large-scale rapid removal of iron from river water is a general phenomenon during estuarine mixing. The river-borne ‘dissolved’ iron consists almost entirely of mixed iron oxide-organic matter colloids, of diameter less than 0.45 μm, stabilized by the dissolved organic matter. Precipitation occurs on mixing because the seawater cations neutralize the negatively charged iron-bearing colloids allowing flocculation. The process has been duplicated in laboratory experiments using both natural filtered and unfiltered river water and a synthetic colloidal goethite in 0.05 μm filtered water. The colloidal nature of the iron has been further confirmed by ultracentrifugation and ultrafiltration. A major consequence of the precipitation phenomena is to reduce the effective input of ‘dissolved’ iron to the ocean by about 90% of the primary river value, equivalent to a concentration of less than 1 μmol per liter of river water.     

Role of Suspended Particulate Matter in Regulating the Behavior of Dissolved Uranium in the Yellow River Estuary

In order to examine the mixing behavior of dissolved uranium (U) in estuaries under different suspended particulate matter (SPM) regimes, three laboratory-based experiments were conducted by mixing seawater with river water containing different concentrations of SPM. Comparing this study with other field and laboratory-based experiments, dissolved U behaved differently depending upon the concentration of SPM. When SPM concentrations are >?0.8 g/L in the Yellow River, desorption/dissolution of U from SPM becomes predominant and dissolved U is enriched relative to the theoretical mixing line. However, when SPM concentrations are <?0.8 g/L, dissolved U behaves conservatively with some extent of removal during estuarine mixing. ²³⁴U/²³⁸U activity ratios were somewhat constant showing no measurable isotopic fractionation during physical mixing and U sorption/desorption to/from particles. Addition of dissolved ²³⁸U desorbed/dissolved from SPM during the annual Yellow River water-sediment regulation scheme (Jun 30th–Jul 14th, 2014) was estimated at 6.4?×?10¹¹ dpm, about 9% of the total riverine flux of dissolved ²³⁸U during that same period. This study represents a contribution to studies of dissolved U in muddy rivers and estuaries throughout the world. Results reported here provide not only a perspective to better estimate U flux from rivers to the ocean but also new insights into better understanding its estuarine mixing behavior and controlling factors.     

Water and Organic Carbon Cycles in Monsoon-driven Humid Tropics of the Western Ghats Mountain Belt,India: Insights from Stable Isotope Approach

The Western Ghats form a major mountain belt, next to the Himalayas, in controlling the flux of water and carbon to the northern Indian Ocean. This study attempts to understand the water and carbon cycles in two humid tropical river basins with its streams originating at higher altitudes of the Western Ghats, India. Water and suspended particulate matter (SPM) were collected on a monthly scale during summer monsoon season (June-September) from Swarna and Nethravati rivers draining into the Arabian Sea. For the source apportionment, samples have been measured for stable isotopes of oxygen (δ¹⁸O) and hydrogen (δ²H) in water and stable isotopes of carbon (δ¹³C_POC) in particulate organic matter (POM) at spatial scale from tributaries and main channel of rivers, and runoff water from agricultural land (dominant paddy field) and forest in the downstream region. The association between δ¹⁸O and deuterium-excess in river water and rain water shows that water in these tropical basins depicts rainout effect of marine source moisture during the onset of summer monsoon. As the monsoon intensifies, the fresher rain water replenishes older water stored previously in sub surface soil layer leading to its flushing into the river during summer monsoon season. Stable carbon isotope ratio and elemental ratio of POM (δ¹³C_POC = -27.1 ± 0.4 ‰ and C/N = 8.1 ± 1.7) in two humid tropical river water during summer monsoon season is an admixture of suspended particulates from runoff water of forest (δ¹³C_POC = - 27.82 ± 0.4 ‰) and agricultural land (δ¹³C_POC = -26.29 ± 0.4 ‰). It is found that δ¹³C_POC shows minimal variability with SPM content and C/N ratio within the same organic carbon pool. The study emphasizes the need to consider the agricultural runoff contribution to the rivers while establishing the global elemental budget and observing the global climate change.     

Freshwater-Saltwater Mixing Effects on Dissolved Carbon and CO<Subscript>2</Subscript> Outgassing of a Coastal River Entering the Northern Gulf of Mexico

The delivery of dissolved carbon from rivers to coastal oceans is an important component of the global carbon budget. From November 2013 to December 2014, we investigated freshwater-saltwater mixing effects on dissolved carbon concentrations and CO₂ outgassing at six locations along an 88-km-long estuarine river entering the Northern Gulf of Mexico with salinity increasing from 0.02 at site 1 to 29.50 at site 6 near the river’s mouth. We found that throughout the sampling period, all six sites exhibited CO₂ supersaturation with respect to the atmospheric CO₂ pressure during most of the sampling trips. The average CO₂ outgassing fluxes at site 1 through site 6 were 162, 177, 165, 218, 126, and 15 mol m^?2 year^?1, respectively, with a mean of 140 mol m^?2 year^?1 for the entire river reach. In the short freshwater river reach before a saltwater barrier, 0.079 × 10⁸ kg carbon was emitted to the atmosphere during the study year. In the freshwater-saltwater mixing zone with wide channels and river lakes, however, a much larger amount of carbon (3.04 × 10⁸ kg) was emitted to the atmosphere during the same period. For the entire study period, the river’s freshwater discharged 0.25 × 10⁹ mol dissolved inorganic carbon (DIC) and 1.77 × 10⁹ mol dissolved organic carbon (DOC) into the mixing zone. DIC concentration increased six times from freshwater (0.24 mM) to saltwater (1.64 mM), while DOC showed an opposing trend, but to a lesser degree (from 1.13 to 0.56 mM). These findings suggest strong effects of freshwater-saltwater mixing on dissolved carbon dynamics, which should be taken into account in carbon processing and budgeting in the world’s estuarine systems.     

Removal of dissolved boron and silicon during estuarine mixing of sea and river waters

The behaviour of dissolved boron and silicon during mixing of sea and river waters has been studied in two surveys of the estuary of the River Alde in Suffolk, England. Removal of approximately 25–30 per cent was found for both elements. This appears to be the first report of estuarine removal of dissolved boron. The extent of removal of silicon in the Alde is somewhat higher than that found in other estuaries.     

Seasonal uranium distributions in the coastal waters off the Amazon and Mississippi Rivers

The chemical reactivity of uranium was investigated across estuarine gradients from two of the world’s largest river systems: the Amazon and Mississippi. Concentrations of dissolved (<0.45 μm) uranium (U) were measured in surface waters of the Amazon shelf during rising (March 1990), flood (June 1990) and low (November 1991) discharge regimes. The dissolved U content was also examined in surface waters collected across estuarine gradients of the Mississippi outflow region during April 1992, August 1993, and November (1993). All water samples were analyzed for U by isotope dilution inductively coupled plasma mass spectrometry (ICP-MS). In Amazon shelf surface waters uranium increased nonconservatively from about 0.01 μg I^?1 at the river’s mouth to over 3 μg I^?1 at the distal site, irrespective of river discharge stage. Observed large-scale U removal at salinities generally less than 15 implies a) that riverine dissolved U was extensively adsorbed by freshly-precipitated hydrous metal oxides (e.g., FeOOH, MnO₂) as a result of flocculation and aggregation, and b) that energetic resuspension and reworking of shelf sediments and fluid muds on the Amazon shelf released a chemically reactive particle/colloid to the water column which can further scavenge dissolved U across much of the estuarine gradient. In contrast, the estuarine chemistry of U is inconclusive within surface waters of the Mississippi shelf-break region. U behavior is most likely controlled less by traditional sorption and/or desorption reactions involving metal oxides or colloids than by the river’s variable discharge regime (e.g., water parcel residence time during estuarine mixing, nature of particulates, sediment storage and resuspension in, the confined lower river), and plume dispersal. Mixing of the thin freshwater lens into ambient seawater is largely defined by wind-driven rather than physical processes. As a consequence, in the Mississippi outflow region uranium predominantly displays conservative behavior; removal is evident only during anomalous river discharge regimes. ‘Products-approach’ mixing experiments conducted during the Flood of 1993 suggest the importance of small particles and/or colloids in defining a depleted U versus salinity distribution.     

A chemical survey of the Mississippi estuary

A “snap shot” survey of the Mississippi estuary was made during a period of low river discharge, when the estuarine mixing zone was within the deltaic channels. Concentrations of H⁺, Ca²⁺, inorganic phosphorus and inorganic carbon suggest that the waters of the river and the low salinity (<5‰) portion of the estuary are near saturation with respect to calcite and sedimentary calcium phosphate. An input of oxidized nitrogen species and N₂O was observed in the estuary between 0 and 4‰ salinity. The concentrations of dissolved NH₄ ⁺ and O₂, over most of the estuary, appeared to be influenced by decomposition of terrestrial organic matter in bottom sediments. The estuarine bottom also appears to be a source of CH₄ which has been suggested to originate from petroleum shipping and refining operations. Estuarine mixing with offshore Gulf waters was the dominant influence on distributions of dissolved species over most of the estuary (i.e., from salinities >5‰). The phytoplankton abundance (measured as chlorophylla) increased as the depth of the mixed layer decreased in a manner consistent with that expected for a light-limited ecosystem. Fluxes of NO₃ ^?+NO₂ ^? and soluble inorganic phosphorus to the Gulf of Mexico were estimated to be 3.4±0.2×10³ g N s^?1 and 1.9±0.2 g P s^?1 respectively, at the time of this study.     

Flood zone biogeochemistry of the Ob River middle course

The flood zone of the Ob River, the largest (in watershed area) river of the Arctic Ocean basin, is tens of km wide and, after the Amazon's Varzea, is the world's second largest flooding territory. To better understand the biogeochemistry of the Ob River and adjacent surface waters, we studied, in May and July 2014, the dissolved and colloidal organic carbon and trace metals in small rivers, lakes, and flooded water bodies connected and disconnected with the mainstream as well as the Ob River itself. All major and trace elements were distributed among two major categories depending on their pattern of dependence on the dissolved organic carbon (DOC) concentration. Dissolved inorganic carbon (DIC), Na, Mg, Ca, sulfate, Sr, Mo, Sb and U exhibited a general decrease in concentration with the increase of the [DOC]. The lowest concentration of these elements was observed in DOC-rich humic, acidic (4.9 ≤ pH ≤ 6.1) upland lakes fed by surrounding bogs. These elements marked the influence of underground feeding in July during summer baseflow, which was most visible in flood lakes in the Ob riparian zone and the Ob River itself. In May, the flood lakes were statistically similar to the Ob River. The elevated concentration of DOC (up to 60 mg/L) in the upland lakes was not correlated with groundwater-related elements, suggesting a lack of significant groundwater feeding in these lakes. In contrast, insoluble, usually low mobile elements (Al, Fe, other trivalent hydrolysates, Ti, Zr, Hf) and some metals (Cr, Zn, Ni, Pb) demonstrated a steady increase in concentration with increasing DOC, with the lowest values observed in the Ob River and the highest values observed in small tributaries and organic-rich upland lakes in July. It follows that these elements are limited by their main carriers – organic and organo-ferric colloids, rather than by the availability of the source, peat and mineral soil or plant litter. While for the majority of non-colloidal, groundwater-fed elements with high mobility (DIC, Na, Mg, Ca, K, Sr…) the small tributaries can be used as representatives of the Ob main stream, this is not the case for low mobility “insoluble” elements, such as Fe, Al, trivalent and tetravalent hydrolysates, and metal micronutrients (Cu, Zn, and Mn). The low soluble elements and divalent metals exhibited a much lower concentration in the river mainstream compared to that in the flood lakes, upland lakes and small rivers. This difference is significantly more pronounced in the baseflow in July compared to the spring flood in May. Presumably, autochthonous processes, such as the photo-oxidation and bio-oxidation of organo-ferric colloids and phytoplankton uptake are capable decreasing the concentration of these elements in the river mainstream.     

Relationships Between Geochemical Parameters (pH, DOC, SPM, EDTA Concentrations) and Trace Metal (Cd, Co, Cu, Fe, Mn, Ni, Pb, Zn) Concentrations in River Waters of Texas (USA)

Water samples from eight major Texas rivers were collected at different times during 1997–1998 to determine the dissolved and particulate trace metal concentrations, expected to show differences in climate patterns, river discharge and other hydrochemical conditions, and human activities along the different rivers. Specifically, two eastern Texas rivers (Sabine, Neches) lie in a region with high vegetation, flat topography, and high rainfall rates, while four Central Texas rivers (Trinity, Brazos, Colorado, and San Antonio) flow through large population centers. Relatively high dissolved organic carbon (DOC) concentrations in the eastern Texas rivers and lower pH led to higher Fe and Mn concentrations in river waters. The rivers that flow through large population centers showed elevated trace metal (e.g., Cd, Pb, Zn) concentrations partly due to anthropogenically produced organic ligands such as ethylenediaminetetraacetic acid (EDTA) present in these rivers. Trace metal levels were reduced below dams/reservoirs along several Texas rivers. Statistical analysis revealed four major factors (suspended particulate matter [SPM], EDTA, pH, and DOC) that can explain most of the observed variability of trace metal concentrations in these rivers. SPM concentrations directly controlled particulate metal contents. Variation in pH correlated with changes of dissolved Co, Fe, Mn, and Ni, and particulate Mn concentrations, while DOC concentrations were significantly related to dissolved Fe concentrations. Most importantly, it was found that, more than pH, EDTA concentrations exerted a major control on dissolved concentrations of Cd and Zn, and, to a lesser extent, Cu, Ni, and Pb.