Quantifying the Effects of Land Use and Erosion
Particle class size analysis sheds light on the relationship between land use and suspended sediment.
Restoration of water quality in impaired watersheds requires understanding the complex interconnections between hydrologic processes, climate, land use, water quality, ecology, and human socioeconomics. Unfortunately, information regarding those relationships is often inadequate or absent. A lack of technology and analytical tools has contributed to imprecise decision-making, resulting in a lack of progress toward identifying and prioritizing mitigation strategies. It is therefore not surprising that intense debates are taking place in communities regarding the magnitude and implications of water-quality problems and the potential for natural resource restoration and sustainability (Law et al. 2008).
Pollution transport from diffuse sources is most often driven by meteorological events (i.e., precipitation). Precipitation events are, therefore, highly correlated to pollutant loadings from a given watershed (Novotny and Olem 1994). The Environmental Protection Agency’s Wadeable Streams Assessment, a biological assessment of 1,392 randomly selected wadeable stream sites in the conterminous US, indicated that 42% of the nation’s wadeable stream length is in poor biological condition relative to reference site conditions (USEPA 2006). Suspended sediment is one of the main causes of water-quality impairment in freshwater systems. Sediment is a naturally occurring and necessary element in stream ecosystems. However, both a lack and an excess of stream sediment can be equally damaging to stream geomorphologic and ecological functions. Streams deprived of sediments can experience increased channel erosion, habitat degradation, nutrient depletion, and significantly altered biological health (e.g., fish populations). For example, increased light penetration due to lowered turbidity can reduce primary productivity, thus breaking down the stream ecosystem food web and giving non-native, sight-feeding fish a competitive advantage over native species (Kondolf 1997). Sediment also provides a valuable nutrient source for invertebrate populations (Dodds and Whiles 2004, Koirala 2009). However, excess fine sediment can fill the interstitial spaces of gravel in spawning beds, reduce available oxygen needed by fish embryos, and cause gill inflammation and eventual death to young and susceptible fish (Kondolf 1997). Therefore, a need exists to better understand natural and anthropogenic sediment loading processes, with particular focus on fine particle classes, especially given the potential for smaller sediments to be transported from expanding urban landscapes.
Figure 1. Locations of five permanent gauging stations in the HWC
Unlike many other water-quality constituents of concern, identification of sediment-impaired watercourses is complicated because sediment in streams and rivers has both natural and anthropogenic sources. Consequently, quantifying the level of impairment and, thus, the efficacy of best management practices is confounded by a lack of understanding of background (i.e., presettlement) sediment loads. In most urban stream systems, land-use changes in the watershed over time have dramatically altered the hydrology and sediment loading to the system. Urban systems are often the least resilient to additional perturbation because of the compounded effects of development. Following construction, impervious surfaces impede infiltration and most rainfall becomes runoff. Natural drainage systems become rapidly overwhelmed, incising the stream channel, eroding banks, and increasing downstream discharge.
Turbidity measurements are the most common means for determining suspended sediment concentration (SSC) (Pruitt 2003). However, accuracy can be much improved by conducting gravimetric analyses of manually collected water samples or by automatic water samplers (Edwards and Glysson 1999, Davis 2005). Gravimetric methods involve filtering the sediment from a known sample volume using a vacuum filtration process (ASTM 2007). Using a series of different sized filters, information about particle size distribution of the sample can be obtained. However, sample collection and laboratory analyses using these methods tend to be prohibitively expensive, complex, labor intensive, and prone to human error (Gray and Gartner 2009).
The relationships between land use, erosion, and sediment loading and transport are not well understood. Hydrologic modification exacted by development can increase or decrease (to varying degrees) erosion from overland sources, suspended sediment loads, and specific particle class concentrations. There is a need to quantify pollutant transport mechanisms and to identify the potential pathways of contaminants dispersal from variable source areas (VSA) to receiving water bodies. There is also a need to better understand the effects of land use on sediment loading, including particle size class (PSC) distribution. Particle size may in part be determined by the type(s) of land use; therefore, some land uses may be more significant than others for water-quality impairment. For example, larger particle size classes may originate primarily from localized riparian development and instream hydrogeomorphological processes (i.e., bank erosion, channel incision), while smaller particle size classes may increase in urban environments that tend to preferentially collect and transport fine-grained particles across a complex, impervious landscape (Hubbart and Freeman 2010). Regardless of the mechanism, there is little argument that freshwater resources are increasingly threatened by sedimentation caused by upland soil and streambank erosion, agriculture, and rapidly spreading urbanization. Further research is needed to enhance understanding of the mechanistic relationships between land use, erosion processes, and relative contributions to suspended sediment loading.
Laser Particle Diffraction
Recent innovations in suspended sediment monitoring include fully automated in-situ devices capable of continuously sensing and logging suspended sediment concentrations and particle size classes. Commercially available instruments include those that use bulk optics, acoustics, pressure differentials, and laser optics. Bulk optics instruments measure the turbidity of the water, where turbidity is an expression of the optical properties of the water that cause light to be scattered and absorbed rather than transmitted directly through solution (Gray and Gartner 2009). Other instruments currently used to quantify SSC include acoustic backscattering technology, portable acoustic Doppler current profilers (ADCP) (Gray and Gartner 2009), and laser diffraction analyzers (Agrawal and Pottsmith 2000).
Laser diffraction instruments measure optical scattering of light over a wide range of angles, providing a multiparameter measurement corresponding to a wide range of particle sizes (Agrawal and Pottsmith 2000). Laser diffraction instruments use specially constructed detectors that detect light-scattering effects of particles of individual particle classes. The total volume of particles is used to estimate total concentration independent of particle density or size distribution. Laser diffraction instruments also estimate mean particle size by computing the ratio of total particle area to total particle volume. Notably, many laser diffraction instruments estimate a volumetric concentration (i.e., μl/l) of sediment as opposed to a mass concentration (Agrawal and Pottsmith 2000), and therefore require estimates of particle density to make the mass conversion to mg/l.
Particle Size Class Detection
Laser in-situ scattering and transmissometry (LISST) instruments were originally developed for marine sediment studies. However, the technology has since evolved for deployment in freshwater systems. The LISST-StreamSide (Sequoia Scientific Inc.) is designed for monitoring sediment in shallow rivers, streams, and ponds. A pump is placed in the water body to supply water to the particle analyzer for analysis. After analysis, the water sample is returned to the source, eliminating the need for handling and lab processing of the sample. The LISST-StreamSide senses particle sizes ranging from 1.9 to 387 μm (note: 1 µm = 3.95 x 10-5 inch). Validation studies indicate that the LISST is able to estimate particle class concentrations with an accuracy of 10 to 20%, with greater accuracy for smaller particle classes (Agrawal and Pottsmith 2000, Gartner et al. 2001). The dynamic range of the LISST is 1:200. Therefore, the largest size that can be measured is 200 times the smallest size. This range is spread over 32 detectors (i.e., size classes). Particle size class detector rings are logarithmically spaced such that each upper size bin is 1.18 times the lower size. For example, the ring that represents 5 µm covers a range of 4.7 to 5.54 µm, while with larger particles, the ring that represents 200 µm covers a range of 179.2 to 211.5 µm. Therefore, the ring that detects larger particle size has a much broader sensitivity range as compared to smaller size classes, thereby reducing resolution and accuracy (Gartner et al. 2001).
The following case study was undertaken to quantify particle distribution during the month of March 2010 under conditions of typical spring rainfall in an Ozark border stream of central Missouri, using a nested-scale watershed study design to test the effects of land use on sediment regime. The LISST-StreamSide provides high-resolution SSC and particle-size distribution data that are not achievable through traditional methods of sediment sampling. Therefore, results were expected to provide newly quantified relationships between rainfall, runoff, particle size class concentrations, and land use in a central Missouri urban watershed. The ability to quantifiably distinguish these relationships by laser particle diffraction would be a truly novel approach to achieving improvements to water quality.
Hinkson Creek Watershed
In the state of Missouri, more than 150 water bodies have been identified as impaired or limited for a variety of beneficial uses since 2000. This figure is 15% higher than the national state average of 25% fresh water impairment. This is particularly significant considering that Missouri is one of nine central US states that contribute more than 75% of upland nitrogen and phosphorus entering the US Gulf of Mexico (Alexander et al. 2008). Studies are therefore warranted in Missouri to improve water quality and thus reduce pollutant contributions to the Gulf. The Hinkson Creek watershed (HCW) is located within the Lower Missouri-Moreau River Basin (LMMRB) in central Missouri (Figure 1). The HCW was instrumented with a nested-scale experimental watershed study design in the fall of 2008 to study contemporary land use and urbanization effects on hydrologic processes, water quality, and localized land-use effects on climate and biological community health. In general, nested watershed study designs use a series of sub-basins within a larger watershed to study environmental variables and partition land-use types to facilitate quantification of land-use effects. Sub-basins are often partitioned based on dominant land use and characteristics of the hydrologic system (Capel et al. 2008). A nested watershed study design enables researchers to quantifiably ascertain the influencing patterns and processes observed at each location (Pickett et al. 1997, Hubbart et al. 2007, Karwan et al. 2007). Each of five fully automated gauging stations monitors stage as well as a complete suite of climate variables. Instrumentation in the HCW is complemented by a United States Geological Survey gauging station (USGS-06910230) that has collected data intermittently since 1966 (Figure 1, site 4).
Hinkson Creek flows through a catchment basin of approximately 81.2 square miles (231 square kilometers). The creek flows approximately (32 miles (52 kilometers) in a southwesterly direction to its confluence with Perche Creek, ultimately flowing into the Missouri River. Elevation ranges from 580 feet (177 meters) at the confluence of Perche Creek to 890 feet (274 meters) above mean sea level in the headwaters. Hinkson Creek is classified as a Missouri Ozark border stream located in the transitional zone between Glaciated Plains and Ozark Natural Divisions (Thom and Wilson 1980). Average annual temperature and precipitation is approximately 57ºF (14ºC) and 38 inches (980 millimeters), respectively. Soil types range from loamy till with a well-developed clay pan in the uplands (Chapman et al. 2002) to thin cherty clay and silty to sandy clay in lower reaches. Urban areas are primarily residential (ca. 90,000 residents) with progressive commercial expansion. Land use for the entire watershed is approximately 34% forest, 38% pasture and cropland, and 25% urban area, with the remaining land area in wetland, open, or shrub/grassland areas. Table 1 lists the land-use area (assuming five land-use divisions) and stream length for each of the nested treatments, as well as the entire watershed.
In 1998, the Missouri Department of Natural Resources (MDNR) identified a portion of the LMMRB as critical for controlling erosion and nonpoint source pollution (MDNR 2006). Watershed restoration efforts in the LMMRB were accelerated by mandates of the Clean Water Act (CWA) and subsequent lawsuits. Hinkson Creek watershed is representative of the LMMRB with respect to hydrologic processes, water quality, climate, and land use and was one of the first water bodies in Missouri to be placed on the CWA 303(d) list of impaired waters.
LISST Analyses Protocol
Two thousand mL grab samples were collected at regular times four days per week (Monday, Wednesday, Friday, and Saturday) at sites 1, 3, and 5 (Figure 1) in the HCW during March 2010 for particle analyses in a typical month of spring rainfall runoff. Samples were collected in an upstream direction from the streambank using a swing sampler to avoid disruption of bottom sediments. All samples were stored at a temperature of 4ºC and processed within seven days of collection. Water samples were run through the LISST in the laboratory. Sampling specifications are easily programmed through a graphics touch display on the LISST. Primary analyses settings include a pre-sampling background check, which allows for automatic calibration of the instrument using deionized or other clean water sources, and sampling duration. For the current work, 30-second sample durations (approximately 0.2 gallons, or 0.6 liters) were used to optimize sampling accuracy. Precipitation and flow were measured at the site 4 climate station and USGS stage-monitoring site. The total contributing area draining to sites 1, 3, and 5 is 28 square miles (77 square kilometers), 44 square miles (114 square kilometers), and 80 square miles (206 square kilometers), respectively. In general, sites 1 and 3 are primarily forested and agricultural/pasture lands, and site 5 is approximately 25% urban, much of which is high-density (approximately 40 to 60% impervious surface) (Table 1).
Figure 2. Precipitation and streamflow (top) and select sediment particle classes (bottom) measured at sites 1, 3, and 5
Results and Discussion
Weather during the month of March 2010 was typical for the central Missouri wet season (Figure 2). Average temperature and average stream flow during the month of March was 45.5°F (7.5°C) and 2.4 m3/s (84 f3/s) respectively. Total precipitation for March was 2.8 inches (72 millimeters). Five appreciable precipitation events occurred during the month, with an average event depth of 0.5 inches (12.4 millimeters). Of the five events, three were larger events concluding on March 10, 21, and 28 with average precipitation of 0.6 inches (16.2 millimeters). These events yielded notable peak flows averaging 576 f3/s and ranging from 251 to 1,010 f3/s (16.3 m3/s, range 7.1 to 28.6 m3/s) (Figure 2, climate graph). Peak flows resulted in two large sediment transport events following the March 10 and 22 precipitation events (Figure 2, sediment graphs).
|Figure 3. Typical post-rainfall event stage on March 28, 2010, at sites 1, 3, and 5 (top to bottom)
Figure 3 shows typical post-storm flow conditions on March 28, 2010, at each of the three nested monitoring sites of the current study. For the month of March 2010, average total concentration of sediment (μl/l) at monitoring sites 1, 3, and 5 was approximately 51, 25, and 27 μl/l respectively (Figure 4). Corresponding mean particle size was approximately 213, 168, and 119 μm respectively. Average total concentration of suspended sediment subsequent to the March 10 and March 22 peak flow events at the three monitoring sites was approximately 83, 41, and 47 μl/l and 47, 56, and 72 μl/l at sites 1, 3, and 5, respectively. It is notable that total sediment concentration was at least twice as high at monitoring site 1 relative to sites 3 and 5 subsequent to the March 10 event, while this relationship was reversed subsequent to the March 22 precipitation event (Figure 2).
There were no appreciable precipitation events prior to the month of March in 2010. Therefore, March weather produced the first large sediment fluxes of the year, providing an opportunity to examine the effects of heavy spring rains on a “primed” system (in terms of sediment) and variation in particle class separations at different locations in the watershed. Results showed a clear flux of sediment, in particular larger particle sizes, at site 1 following the March 10 precipitation event. The magnitude of the site 1 sediment flux subsequent to the March 10 event was twice the sediment flux concentration at sites 3 and 5. Whether this is due to land use or varying precipitation regime throughout the watershed provides impetus for future work in the watershed. However, since the sediment load did not occur at sites 3 and 5 subsequent to the first major precipitation event, it is conceivable that larger particles settled from suspension prior to reaching sites 3 and 5, which are 7.5 And 18 miles (12 and 29 kilometers) downstream, respectively, from site 1. This may also help to explain the sediment flux observed subsequent to the March 22 precipitation event, during which a larger fraction of sediment was detected at site 5.
Figure 4. Mean concentration and mean particle size for March 2010
Given sufficient time and runoff volume, larger sediment particle classes will be transported out of the watershed. However, it should also be borne in mind that larger sediment may be eroded in transit, thus being reduced in size and potentially detected as smaller particles at the watershed terminus. Despite clear trends in the results, precise quantifiable variation in particle analyses, including total concentration and particle size classes, cannot be achieved with the sampling regime used in the current work. The findings here, however, point to the potential for the LISST to provide new information that could be improved with higher-resolution data (e.g., daily or hourly).
Total sediment concentrations were higher at monitoring site 1 subsequent to the March 10 precipitation event. In general, at sites 3 and 5, the ratio of smaller size classes to larger size class particles decreased such that total suspend sediment was primarily composed of finer particles at site 5. This previously unquantified information may be attributed to either naturally occurring or anthropogenic effects, discussed previously in this article. However, exacting the mechanisms associated with these results is beyond the scope of the current analysis and requires evaluation of higher-resolution time-series data over a longer period of time. Figure 5 reflects a cube-root transform of average monthly data from each monitoring site. It is of interest to note the gradual change from a higher number of larger particle size classes at site 1 to a higher concentration of smaller size classes at site 5. This relationship may be better visualized as percent of total concentration by the lumped particle size classes shown in pie charts (Figure 6). Figures 5 and 6 show a clear change in particle size classes progressing downstream from the headwaters. Hubbart and Freeman (2010) describe the process whereby suspended sediment originating from overland sources enters a stream system and either remains suspended and is thus rapidly transported out of the watershed, or settles out of suspension, having exceeded the transport capacity of the stream. In the latter case, sediment may be deposited in the streambed or as alluvium, where it will remain until it is mobilized during a storm event of sufficient volume. This process is repeated until the sediment is either transported from the watershed or removed from the stream system by other means (e.g., gravel extraction). There is little doubt that similar processes are at work in the HCW.
Figure 5. Average particle class concentration pre- and post-storm of particle class size concentrations (log10) shown by cube-root transform
As expected, there is a dramatic increase of PSCs with increased flow (Figure 2). However, concentrations of fine particles are on average quantifiably higher relative to large particle classes at sites 3 and 5 relative to site 1 (Figures 5 and 6). Further, the ratios of large to small size class concentrations appear to be negatively correlated between land-use types and flow events, depending on the location in the watershed (e.g., forest or agriculture land use versus urban). These ratios have not been previously quantified in the Midwest, further illustrating the potential of the LISST to significantly improve the knowledge base of sediment and particle size class transport processes in the region.
Figure 6. Pre- and post-storm lumped particle class concentrations presented
as percent of total from grab samples
Table 1. Total and cumulative contributing (km) and land-use (%) areas and stream length for five monitoring sites
Figure 6 shows that the largest particle classes (lumped: 216.66 to 356.79 µm) comprised almost 80% of the total concentration of suspended sediments at site 1, whereas this size class comprised only 58% (on average) of the total concentration at site 5. At site 5, smaller PSCs comprised almost 40% of the total concentration. This further emphasizes the points made previously by Hubbart and Freeman (2010) and points to a potential punctuated equilibrium of sediment flux in this system, such that the movement of sediment may depend on event magnitude and the ability to transport the sediment. The sediment will therefore settle until subsequent events of sufficient magnitude to move it occur. This is well illustrated in Figure 7, which shows the percent change of average PSC concentration between the sites further indicating a slow transition from larger particle classes at site 1 to smaller particle classes at site 5. It further implicates land-use effects, such as those produced by agriculture and/or greenfield (i.e., undeveloped) development, which may incite erosion of larger particle classes relative to urban and redevelopment lands, which preferentially diffuse smaller particle classes due to preexisting compaction and increased impervious surface cover. This finding holds important implications, as smaller particle classes could instigate a greater degree of stream ecosystem damage compared with larger particle classes.
Figure 7. Percent change in average particle class concentrations
for March 2010
A basic assumption of LISST onboard computations is that all particles are homogeneous spheres. This may not hold true, especially in soils with a large silt or clay component, which are, on average, not spherical. Further, while the LISST-StreamSide measures particle sizes between 2.5 and 500 μm (Agrawal and Pottsmith 2000), smaller particles (i.e. <2.5 μm) may aggregate into flocs, producing a signal in larger PSCs and potentially skewing results. Similarly, larger particle classes may represent lightweight flocs or biological material. Further, during high flows it is possible to get larger sediment particles in suspension. The same is true during slow flows, but the larger particles may not be heavy grains of sand that are unable to stay in suspension. Finally, while volumetric estimates of sediment concentrations are useful, volumetric indices are not the conventional unit for presenting sediment concentrations. In general, lacking in sample-specific information, the conversion from μl/l to mg/l is conducted assuming a particle density of 2.65 g/cm3 (i.e., the density of silica). However, it is likely that mass concentrations calculated in this way will not correlate well to traditional gravimetric estimations. It is therefore advisable to calibrate laser diffraction technology with gravimetric methods. Gravimetric calculations of sediment were not conducted for the current work. However, estimating particle densities for each of the 32 size classes of the LISST will be the objective of future work in the Interdisciplinary Hydrology Laboratory of the University of Missouri web.missouri.edu/~hubbartj. In spite of potential complications, instrument configuration can generate data that have the potential to improve the mechanistic understanding of suspended sediment concentrations and yield in freshwater supplies. LISST technology may therefore advance the current level of understanding of suspended sediment and particle size concentrations across a wide variety of land-use types and runoff events, facilitating a greater understanding and, therefore, improved management of freshwater resources.
This case study is among the first quantification of sediment transport by particle size class from a dynamic urbanizing watershed in the central US. Watershed studies, such as that of the HCW, utilizing a nested-scale study design can provide novel information to watershed managers, enabling them to make science-based decisions to meet local and national water-quality standards. This study demonstrated that during typical March rainfall in central Missouri, almost twice as much sediment was transported from the headwaters to a point approximately 40 kilometers downstream. On average, particle classes transported were almost twice as large (2.0 to 500 µm size range) in the headwaters, where land use is primarily forest and agriculture. Eighty percent of the total sediment at the headwaters site (site 1) was between 216 and 357 µm, while that number was reduced to almost 40% at site 5, approximately 40 kilometers downstream.
There was an apparent event-based flux, or pulse, to sediment transport in the HCW. These results illustrate the need to better understand fine-particle suspended sediment concentrations in stormwater runoff from urbanizing systems relative to that from agricultural and/or forest environments. Further, these results emphasize the probability of a proposed sediment transport punctuated equilibrium as opposed to a continual steady-state, gradual flux. These relationships in the HCW likely persist in similar systems in the central US. Continued work may therefore hold important implications for understanding the effects of urbanization on sediment loading, instream biota, and sustainable water-quality- and water-resource-related commodities. The LISST-StreamSide holds promise for helping to achieve these goals. However, validation using gravimetric methods is strongly advised.
In watersheds the size of the Hinkson Creek watershed, comprehensive management approaches that examine not only the volume of water causing impairments and the variable use landscape but also the pollution load being transported are imperative. In the HCW and other similarly affected watersheds across America, the work presented here is timely given federal legal mandates to provide quantifiable estimates of total maximum daily loads (TMDLs). Given the range of land uses and scale of the watershed, the HCW serves as common ground encouraging cooperation, trust, and innovation among watershed stakeholders to reach a common goal to improve and sustain water quality, and as a model urban watershed for similar studies.
Author's Bio: Jason A. Hubbart is an assistant professor of forest hydrology and water quality at the University of Missouri and director of the university’s Interdisciplinary Hydrology Laboratory in the Department of Forestry.
Author's Bio: Naomi Gebo is a first-year doctoral student in the Interdisciplinary Hydrology Laboratory in the Department of Forestry at the University of Missouri.
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