Environ. Sci. Technol. 1997, 31, 3218-3225
Organic Carbon and Nitrogen Content Associated with Colloids and Suspended Particulates from the Mississippi River and Some of Its Tributaries COLLEEN E. ROSTAD* AND JERRY A. LEENHEER U.S. Geological Survey, Mail Stop 408, 5293 Ward Road, Arvada, Colorado 80002 STEPHEN R. DANIEL Chemistry and Geochemistry Department, Colorado School of Mines, Golden, Colorado 80401
Suspended material samples were collected at 16 sites along the Mississippi River and some of its tributaries during July-August 1991, October-November 1991, and AprilMay 1992, and separated into colloid and particulate fractions to determine the organic carbon content of these two fractions of suspended material. Sample collection involved centrifugation to isolate the suspended particulate fraction and ultrafiltration to isolate the colloid fraction. For the first time, particulate and colloid concentrations and organic carbon and nitrogen content were investigated along the entire reach of the Mississippi River from above Minneapolis, Minnesota, to below New Orleans, Louisiana. Organic carbon content of the colloid (15.2 percent) was much higher than organic carbon content of the particulate material (4.8 percent). Carbon/nitrogen ratios of colloid and particulate phases were more similar to ratios for microorganisms than to ratios for soils, humic materials, or plants.
Introduction Distinction between particulate and colloidal phases is important in riverine geochemistry assessments because transport of these two phases is different, the particulate phase moves by a series of alternating deposition and suspension events, subject to pulses and ebbs of natural river hydraulics. However, colloidal particles are too small to settle out, and will provide more consistent movement of colloid-associated compounds than the particulate phase (1). Transport and deposition of the particulate phase in the Mississippi River, the world’s seventh largest, has been studied in some detail (2, 3). Areas of the Mississippi River where the slowing river has consistently dropped its suspended particulate load include Lake Pepin (below the Twin Cities) and Pool 19 (near Keokuk, IA) on the Upper Mississippi River (4). On the lower Mississippi River, areas in Louisiana involve extensive deposition and resuspension cycles (5). Distinction of phases is also important to river transport models. Organic contaminants in a river system can be located in any of several compartments, such as water, particulate, colloid, bed sediment, or benthic plants and * Author to whom correspondence should be addressed. Fax: (303) 467-9598. E-mail:
[email protected].
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organisms. Partition coefficients enable prediction of behavior of an organic compound in the environment by indicating to what extent the compound at equilibrium tends to be partitioned into (or associated with) aqueous or various solid phases. Partition coefficients determined in laboratorycontrolled systems at equilibrium can differ from partition coefficients determined in the natural environment and may reflect an inadequate accounting of the colloid phase. Several factors have been employed to justify these deviations: dependence on suspended sediment concentration (6-8) or presence of colloids (9-12). Colloid and particulate phases differ in organic carbon content, a critical distinction since partitioning of nonionic lipophilic organic chemicals to sediments is dependent upon the organic carbon content of the solid phase (13, 14). Although the importance of the colloid phase in understanding environmental fate and transport of contaminants has been recognized (9, 11, 15-19), specific information has been scarce. Organic carbon contents of the colloid phases have been calculated in several studies (11, 20) and inferred in others (12, 21). Some studies have attempted to quantify the colloid phase by dry weight, dissolved organic carbon (DOC), or turbidity measurements (11, 15, 18, 21). Particulate and colloidal material was isolated as part of a multidisciplinary study of the Mississippi River by the U.S. Geological Survey (22-28). Sixteen sites from near Minneapolis, MN, to south of New Orleans, LA, including four tributaries, Minnesota, Illinois, Missouri, and Ohio Rivers, were sampled during cruises in July-August, 1991, OctoberNovember, 1991, and April-May, 1992. Summer and fall cruises were made during periods of low flow, in contrast to the spring cruise, which was made during high-flow conditions on the Mississippi River below Keokuk, IA. The distinction between particulates and colloids will always be somewhat operational in nature (12). Colloid size cannot be inferred from filter pore size and must be determined by other means (29). Because this separation is operationally dependent, the isolation technique for the particulate and colloid phases including colloid particle sizes were previously described in detail (30). Previous studies indicated that water, suspended sediment organic carbon, and dissolved organic carbon loads were relatively invariant throughout the year in the Mississippi River, with suspended organic carbon load being slightly greater than the DOC load, except for low-flow periods in late summer (31). Although initial colloid studies provided some information (30, 32), transport of organic carbon and nitrogen associated with Mississippi River colloid in comparison to the suspended particulate phase was unknown. Consequently, an assessment of the distribution of organic carbon was needed to provide a basis for evaluating current organic carbon and nitrogen transport and possible pollutant transport associated with suspended material. This paper describes seasonal variations and factors affecting transport of organic carbon and organic nitrogen of suspended material (colloid and suspended particulate phases) at selected sites on the Mississippi River and some of its tributaries.
Experimental Methods Colloid and particulate samples were isolated from whole water samples collected at sites shown in Figure 1. Downstream of the Mississippi and Missouri river confluence reflects a free-flowing river. Above that confluence, the river is essentially a lock and dam system, which impounds the river into a series of lakes (33). These two reaches of the river may have significant differences in particulate and colloid distributions.
S0013-936X(97)00196-X CCC: $14.00
1997 American Chemical Society
in less than 2 L of liquor, preserved with chloroform, freezedried, and a portion taken for organic carbon and nitrogen analysis. Colloid particle size ranged from 360 to 3500 and averaged 1100 nanometers (30). Particulate and colloid organic carbon and nitrogen contents in percent dry weight were determined by Huffman Laboratories, Golden, CO (36). Nitrogen was not speciated due to its low concentration, and because organic nitrogen is the dominant form in river suspended material (37). Relative percent difference of six duplicate analyses averaged 0.66% for colloid organic carbon, 12% for particulate organic carbon, 0.79% for colloid nitrogen, and 20% for particulate nitrogen. Ash content was not determined; therefore, these organic carbon and nitrogen values are not ash-free elemental compositions.
Results and Discussion
FIGURE 1. Map showing location of sample sites along the Mississippi River and some of its tributaries (adapted from ref 1). The particulate and colloid isolation technique is described in detail elsewhere (30, 34, 35). Briefly, at each sampling site, the research vessel was positioned, and a discharge-weighted water sample was pumped at each vertical of the river crosssection. River water was pumped into a 63 µm sieve and flowed into a continuous-flow, high-speed centrifuge, operated at 16 000 rpm [relative centrifugal force (RCF) of 15000g] at 2 L/min flow rate. The smallest particle size retained in the centrifuge used in this field work was calculated (as spherical clay particles) to have an effective diameter of about 370 nm (35). Suspended particulates were deposited on the cylindrical wall of the centrifuge bowl that was lined with a removable Teflon sheet, which was transferred into a Teflon bag, where the sediment sample was resuspended using gentle massage. Suspended particulates from 500 to 1000 L of river water collected during sampling were then concentrated in 1-2 L of solution, which was preserved with chloroform, allowed to settle for 1 week, aspirated to remove supernatant, air dried, and a portion taken for organic carbon and nitrogen analysis. Colloids remaining in the aqueous phase after centrifugation were isolated using a tangential-flow ultrafiltration system with a Teflon bellows pump for solution circulation. The centrifuge eluate was processed through 30000-molecularweight (0.005 µm) membranes, which were then placed in PFA Teflon bags and colloids resuspended off the membranes. The colloids from 500 to 1000 L of river water were contained
Colloids and Particulates: Concentration. Particulate and colloid concentrations and ancillary hydrologic data for all three cruises are listed in Table 1. Concentrations of colloids and suspended particulates at the 16 sites along the Mississippi River during July-August, 1991, indicate that the Minnesota River contributed a significant portion of suspended particulates in this area of the river. Particulate concentration decreased dramatically as the river passed through Lake Pepin, a long lake commonly described as a sediment trap. The particulate concentration near Pepin, WI (5.6 mg/L), decreased to only 8% of the concentration upstream at Hastings, MN (66.1 mg/L), while the colloid concentration only decreased to 58% (3.21 and 5.53 mg/L, respectively). Compared to the Mississippi River at sites above their confluences, the Ohio River had lower concentrations of both particulates and colloids, and the Missouri River had higher particulate concentration. Colloids averaged 10.1% by weight of the total suspended material (colloids and particulates) for most of the river in summer. Particles less than 2 µm in these Mississippi River samples were found to contain smectite, illite, kaolinite, and quartz (25, 38). Detailed mineralogy of the Mississippi River particulate phase is available elsewhere (39). Concentrations of colloids and suspended particulates in October-November, 1991, samples indicated trends similar to those observed in the summer. In the fall samples, the proportion of colloid phase by weight increased compared to the summer samples at every site, and exceeded 30% at two upper river (above St. Anthony Falls and near Pepin) sites. The colloid phase averaged 14.6% by weight of total suspended material. During April-May, 1992, the distribution of suspended material in the river changed (compared to summer and fall samples) due to spring runoff conditions. The Winfield site was near flood stage. Spring runoff from the Missouri River produced the highest suspended sediment concentration of ten cruises (40-42). The stage of the Ohio River was falling; hence, its concentration of suspended material was lower than the Mississippi River at Thebes, IL, above their confluence. The percentage of colloid phase by weight of the total suspended material was greater than 10% above Winfield, MO (except for the Minnesota River) but decreased in the lower river below the Mississippi and Missouri river confluence. Using this centrifugation and ultrafiltration isolation technique, colloids averaged about 12% by weight of the total (
