New Mexico Water Science Center Sediment Laboratory
The U. S. Geological Survey (USGS), New Mexico Water Science Center (NMWSC), works in cooperation with State and Federal agencies in New Mexico to provide reliable and impartial sediment data and interpretation to resource managers, planners, and other stakeholders, and the general public. The NMWSC has been collecting daily suspended sediment samples and periodic turbidity, bed material, suspended sediment samples on the major rivers of New Mexico since about 1942. The NMWSC Sediment Laboratory routinely performs sediment analyses for suspended-sediment concentration, determines percentages of sand and finer materials, and completes particle-size analyses for suspended and bed-material sediment. USGS Sediment Data Portal
New Mexico Sediment Program
The first gaging station established by the Geological Survey in the United States was on the Rio Grande at Embudo Jan. 1, 1889. Cooperation with Territory of New Mexico began about 1907, and Territory or State cooperation in varying amounts continued until 1915, the work being directed from the Denver office. From 1916 to 1930 the State conducted its own stream-gaging program. In 1931 a new State cooperative program was begun and a Geological Survey district office established in Santa Fe. Agreements have also existed with county, municipal, and private organizations, and with other Federal agencies.
The NMWSC has been collecting suspended sediment data on the major rivers of New Mexico since about 1942. Sediment data are collected using cross-sectional collection techniques or by automated point samplers. The NMWSC Sediment program is currently collecting and analyzing daily suspended sediment samples and periodic turbidity, bed material, suspended-sediment samples at many sites on the Rio Grande and its tributaries. The results of the daily suspended sediment samples are analyzed along with streamflow record to determine daily values of suspended sediment load. This data are used by State and Federal agencies for many purposes including reservoir management, channel geomorphology, endangered species habitat improvement, and river restoration.
Sediment Characteristics
Sediment is fragmental material transported by, suspended in, or deposited by water or air, or accumulated in beds by other natural agents. Sediment particles range in size from large boulders to colloidal-size fragments and vary in shape from rounded to angular. They also vary in mineral composition and specific gravity. Sediment is derived from a parent material subjected to erosional processes by which particles are detached and transported by gravity, wind, water, or a combination of these agents. When the transporting agent is water, the sediment is termed fluvial sediment”. The U.S. Geological Survey (USGS) defines fluvial sediment as fragmentary material that originates mostly from weathering of rocks and is transported by, suspended in, or deposited from water (Glysson, 1989).
Functions of a Sediment Laboratory
A sediment laboratory in the Water Resources Division of the U.S. Geological Survey has two principal functions: (1) the determination of suspended-sediment concentration of samples collected from streams and (2) the determination of particle-size distribution of suspended sediment, streambed material, and reservoir deposits. Other kinds of analysis, usually associated with these two functions, may include quantitative determination and (or) removal of organic matter and dissolved solids, the specific gravity of sediment particles, the specific weight of soils and sediment deposits, particle shape determinations, and mineral analyses.
Data from the concentration and particle-size determinations are used to make computations of suspended-sediment discharge, computations of total sediment discharge, and computations of the probable volume and specific weight of exposed and submerged reservoir deposits. These are but a few of the many uses made of concentration and particle-size data of fluvial sediment. The reliability of these computations and the utility of the data are dependent on the accuracy and reliability of the laboratory analyses (Guy, 1969).
Suspended-Sediment Concentration
- Evaporation Method
The evaporation method consists of allowing the sediment to settle to the bottom of the sample bottle, decanting the supernatant liquid, washing the sediment into an evaporating dish, and drying it in an oven. The primary disadvantage of the evaporating method is that a correction for dissolved-solids concentration may be required.
- Filtration Method
The filtration method for the determination of suspended-sediment concentration utilizes a filtered crucible, which is a small porcelain or glass cup of approximately 25 ml capacity with a perforated bottom. The crucible is easily adapted to an aspirator system and vacuum filtration. This method eliminates the need to decant the supernatant liquid and eliminates the need for a dissolved-solids correction. A disadvantage of this method is that large quantities of sediment in the sample can clog the filter, which slows filtration.
Particle-size Distribution
- Sand/Fine Method
Sand/fine separations are used to determine sample concentration and the amount of material that is less than or greater than san size. The term fine fraction refers to particles that pass through a 0.062-mm mesh sieve, and sand fraction refers to particles large enough to be retained on such a sieve.
- Sieve-Pipet Method
The pipet method is used to determine particle-size gradation for sizes finer than 0.062mm. The fundamental principle of the pipet method is to determine the concentration of a suspension at a predetermined depth as a function of settling time. Particles having a settling velocity greater than that of the size at which separation is desired will settle below the point of withdrawal after elapse of a certain time. The time and depth of withdrawal are predetermined by Stokes law.
Project Overview
The U.S. Geological Survey (USGS) operated laboratories for the analysis of physical characteristics of natural water bodies, including fluvial sediment. Sediment laboratories producing data for the Water Resources Division of the USGS determine suspended-sediment concentration (SSC), fine/sand separation, and particle size distribution (PSD). The usefulness of these data is dependent on the accuracy and reliability of laboratory analyses. To ensure the physical sediment data produced or used by the USGS are of a known quality and are sufficient to provide long-term comparability and consistency on a national basis, the USGS initiated the Sediment Laboratory Quality Assurance (SLQA) Project in August 1996. To improve and assess data quality produced by the USGS and to enable comparison among laboratories, participation in the SLQA Project by USGS sediment laboratories and by laboratories contracted by the USGS is required. Results from the SLQA studies may be used to assess variability in environmental data and to improve laboratory performance.
Annually, two single-blind SLQA studies are conducted. Participating laboratories are asked to determine suspended sediment concentration, sediment mass, separation of fine- and sand-size material, and particle size distribution using the standard techniques with which they analyze environmental samples. Laboratories are asked to only submit results for analyses they routinely perform. Twenty-six federal, state, contract, and private laboratories participate in the SLQA. Sediment Laboratory QA data may be accessed through this web site.
The SLQA project also prepares double-blind reference samples. This enables the submission of disguised quality control sediment-water samples with environmental samples to laboratories for analysis. The quality control samples are formulated with similar characteristics as the environmental samples with which they are submitted. Analyses of double blind samples provide a measure of bias and variability due to laboratory methods, which may be used to quantify data quality for the accompanying environmental samples. Double-blind samples have also been used for comparing analytical techniques and improving collection methods.
SUMMARY (SLQA)
The U.S. Geological Survey (USGS) is the leading agency in assessing the Nation’s water quality. Sediment is the largest contaminant of surface water by weight and volume, and sediments and sedimentary processes along with human influences can have substantial long-term effects on the biological community’s aquatic habitats, such as spawning beds. Determining the prevailing hydrological conditions that lead to sediment erosion, transportation, or accumulation requires continuous monitoring and sampling. It is important for the USGS to have the ability to compute the suspended-sediment and total-sediment discharge and the probable volume and specific weight of exposed and submerged river and reservoir deposits. To be able to study sediment effectively, the USGS operates sediment laboratories that determine the suspended-sediment concentration and sand/fine separations of fluvial samples. In 1996 the USGS began the Sediment Laboratory Quality-Assurance (SLQA) project to provide information on the bias and variability of the data received from its sediment laboratories.
For each participating laboratory, percent differences for suspended sediment concentration, mass of fine- and sand-size material, and net sediment mass were tabulated for the seven studies completed between August 1996 and July 1999. The median differences between the reported and known values can be used as an indication of laboratory bias. For example, if a laboratory showed no bias, then the median percent difference for that laboratory would be zero. From the first seven SLQA studies, the overall median percent difference for suspended-sediment concentration on an individual laboratory basis showed a negative bias that ranged from –6.80 to –0.34 percent. There was less variability in the median difference for the measured mass of fine-size material than for sand-size material. The overall median percent difference for the mass of fine-size material ranged from –9.21 to –3.35 percent, compared to the mass of sand-size material, which ranged from –0.92 to 28.93 percent.
In 1999, the SLQA project was expanded to include transboundary studies and a new double-blind study. The participation of sediment laboratories associated with other Federal agencies and laboratories in Canada provide an additional benchmark for assessing the performance of USGS laboratories. The double-blind study was introduced to supplement information on the bias, precision, and accuracy of suspended-sediment data provided by the current single-blind studies.