Developing Water Quality Criteria for Suspended and Bedded Sediments
Introduction
Excess fine sediment in streams is a human-caused issue that has likely been around a very long time and it isn’t going away soon. As urban and suburban populations continue to grow and agriculture needs increase, erosion and runoff in our country’s streams will only continue to escalate unless drastic steps are taken. Unfortunately, this will not be beneficial to local communities of aquatic wildlife. There are scientific articles dating back as far as seventy-five years that associate excess sediment with impairment to aquatic wildlife (i.e. Ellis 1936). The Clean Water Act of 1972 was created specifically to fix and maintain integrity of our nation’s water supply and “to provide, wherever attainable, water quality for the protection and propagation of fish, shellfish, and wildlife.” Ambient water quality standards have been put in place for parameters such as dissolved oxygen, bacteria and ammonia, but not sediment. Excessive, erosion, transport, and deposition of sediment are among the leading causes of impairment of habitat and water quality in streams and rivers throughout the United States (USEPA 2006). Together, excess sediments and nutrients account for more kilometers of impairment in assessed streams than anything else (Paul et al. 2008). Comparing reference yields of sediments to respective watershed yields, Simon and Klimetz 2008 showed that many states were outrageously over median values for stable streams. Iowa streams exceeded the median value for stable streams by 243%, while New York was 290% above, Mississippi was 630% above, and Oklahoma was between 2,120% and 7,410% above. So, why haven’t standards been set up for sediment? “Although the U.S. Environmental Protection Agency (EPA) has a mandate under Clean Water Act Sections 104 and 304 to develop sediment criteria, the agency has only recently focused attention on sediment impairments in aquatic ecosystems (Bryce 2008).” Many states have set standards for sediments, but few are consistent (Berry et al. 2003). Criteria will need to involve identification of biological effects and then organism responses to those effects. The focus of this paper is to first identify causes to how and why excess sediment fines cause biological impairment to stream ecosystems, and then to address the need for the development of biologically based sediment criteria for streams. With biologically based sediment criteria, managers should be able to better identify impaired reaches and create remediation plans in given reaches of streams that best benefit the local aquatic fauna they wish to protect.
What is fine sediment and where does it come from?
Paul et al. 2008 defined sediments (in the sense of aquatic biological pollutants) as organic and inorganic particles that are suspended in, carried by, or accumulate in waterbodies. Frequently the terms “fines” and “sedimentation” are used in their broadest sense by freshwater scientists (Wood and Amritage 1997). However, it seems to be common practice to consider particle sizes of 0.06mm or less “silt fines” in streams (Bryce et al. 2008, Bryce et al. 2010).
The supply of sediment from channel sources is strongly related to stream discharge and stability of the channel bed and banks. Sources include river banks subject to erosion, mid-channel and point bars subject to erosion, fine bed material stored in the stream bed, natural backwaters and fine particles trapped in aquatic macrophytes or vegetation. On the other hand, the supply of sediment from non-channel sources may be highly unpredictable depending on its source and mode of transport in a given stream. The sources of these sediments are exposed soils subject to erosion that is then transported through runoff, mass failures within the catchment such as landslides, urban areas that increase volume of runoff and erosion and litter from terrestrial sources such as trees (Wood and Armitage 1997).
From cropland alone, approximately 17 tons ha-1 yr-1 of soil are eroded in the U.S. from combined water and wind causes, and an estimated 60% of this tonnage is deposited in streams and rivers (Pimentel et al., 1995). This seems like much more than streams in a pre-disturbed natural state would ever have needed to deal with. Also, forest removal can increase erosion inputs. Dale Jones et al. 1999 showed habitat diversity decreased and riffles became filled with fine sediments as upstream deforested patch length increased. They concluded riparian buffer length in forested areas should be given strong concentration in stream protection plans.
Sediment obviously has plenty of ways to find its way into streams. Streams are built to naturally deal with this and even require this on smaller scales in undisturbed states. Unfortunately, by channelizing streams, building dams to lessen the effects of flooding, destroying riparian buffers, and increasing runoff, humans have inhibited many streams to function as nature intended.
How does sediment affect aquatic fauna and biological processes?
Detrimental effects from excess sediment in streams are numerous for aquatic organisms. From primary production to macroinvertebrates and vertebrates, suspended and bedded sediments have been shown to have direct and indirect effects on organisms’ abilities to survive and function properly. In the Cenozoic, North America was covered by forests (Williams 1989). Due to this, stream species throughout the continent likely evolved to live in forested watersheds with many riparian buffers to high loads of sediments. Now that humans arrived, these species are living in a variety of forested and unforested streams. “High and sustained levels of sediment may cause permanent alterations in community structure, diversity, density, biomass, growth and rates of reproduction and mortality. Impacts on aquatic individuals, population, and communities are expressed through alterations in local food webs and habitat (Henley et al. 2000).”
Primary production can be greatly affected by suspended sediments as it prevents light from reaching the algae, plants and phytoplankton that rely on it to grow. This in turn can lead to bottom up effects on zooplankton and macroinvertebrates that rely on the primary production for food. Decreases in these, obviously, could lead to decreases in food availability for vertebrates.
Macroinvertebrate assemblages can be greatly affected by excess sediments in the in-stream channel (Mebane 2001). EPT (Ephemeroptera, Plecoptera, Trichoptera) taxa richness is commonly used as a response measure to stream health. These creatures provide many benefits to stream ecosystems such as breaking down leaf litter, scraping periphyton off of substrate and provide fish species with food. A recent study showed fine sediment addition by researchers to cause quick reductions in moss cover and EPT richness in four separate reaches of streams in New Zealand compared to control reaches. (Mathaei et al. 2006). Mollusks may also be affected as the benthic environment is covered with layers of fine sediment. Ellis 1936 showed that 0.6 to 2.5 cm of silt caused significant mortality in one species of mussel. Studies on ubiquitous taxonomic groups such as crayfish and snails are mostly undocumented in scientific literature. Indirect effects to food for either of these taxa would likely lead to their decline (Henley et al. 2000).
Many streams fish studies have indicated detrimental effects from sedimentation. Trout growth may be reduced by suspended sediments as they directly alter behavior or stress fish in a way that reduces foraging activity and prey capture success. It was shown that trout observed feeding in channels prior to addition of sediment were noticeably more reactive, moving between cover objects and feeding areas more frequently and were exposed to predators for longer than control fish (Shaw and Richardson 2001). When deposited in riverbeds, fine sediment can reduce survival of embryos and emergence of fry from reds by decreasing dissolved oxygen and water exchange and trapping emerging fry (Chapman 1988). Juvenile salmonid growth and survival have also been linked to even small increases in sediment concentrations and it is suggested that any reduction of fine sediment in streams could benefit salmonid restoration (Suttle et al. 2004). Fish can’t see to feed if the sediment is suspended and in too high of a concentration. Abundance of fish that are benthic insectivores and herbivores has also been shown to be reduced as the percent of fine substrate increased (Berkman and Rabeni 1987). Fish requiring larger substrates in fast flows (riffle dwelling species) are going to be greatly impacted if fine silts fill up the interstitial habitat.
These and many other reasons are why our country’s stream managers will require national ambient water quality criteria for sediments to apply to watersheds for which they are supposed to be protecting. Biologically-based sediment criteria would indicate biological data are used to set sediment criteria that protect and maintain populations of these native, sediment-sensitive species. They would assume that a level of fine sediment accumulation exists beyond which assemblages of organisms are no longer able to sustain themselves (Bryce et al. 2008).
EPA Suspended and Bedded Sediments Framework and its Benefits
The EPA in 1999 stated that suspended and bedded sediments (SABS) are natural components of aquatic systems and do not cause adverse effects unless they are present in excessive or deficient amounts. When they are in excess there needs to be criteria created to quantify what levels need to be maintained to ensure biological integrity. The SABS framework created by the EPA in 2006 uses an ecological risk assessment approach and compares estimated effects with different candidate criteria values. These values come from information from multiple studies and data sets and use a variety of statistical methods. This framework may be applicable to selection of numeric targets for total maximum daily loads to adopt sediment standards in impaired waterbodies (Paul et al. 2008). The SABS framework provides a scientifically backed approach for detecting effect thresholds that is useful for nontraditional modes of action and risk. This is exactly what stream managers have needed, as certain wildlife react to nontraditional stressors like sediment. Quantifying minimum-effect thresholds is now possible with a variety of methods. Using a variety of methods gives a range of values that a particular method might not be able to attain. This method sets the stage for regulatory authorities to use scientifically derived knowledge about the effects of SABS to guide selection of ambient water quality criteria. Ecological knowledge gained during criteria development is also valuable for determining causes of biological impairments, developing risk estimates, and estimating necessary reductions to restore ecosystem function (Paul et al 2008).
Berry et al. 2003 noted some important issues to consider with using national criteria for SABS. They find that generalizations can be difficult because biological response to both increased suspended sediment and increased bedded sediment varies with species and sediment characteristics. Secondly, they found that after additional research it may be possible to develop national scientifically-defensible SABS criteria using the traditional “toxicological” does-response approach, but these criteria would have to incorporate some habitat-specificity in order to be widely applicable. Next they conclude some habitats that have not been well studied deserve more study, especially those habitats with moderate and variable amounts of SABS. Lastly, they note that many states have set criteria for SABS, but there is little consistency among them. West Virginia, for example, states criteria for increases in turbidity (percent light through water) from point and non-point sources in state waters (West Virginia DEP 2010). In contrast, Oregon is in the process of defining excessive sedimentation criteria based on percent fines data collected as part of the Environmental Monitoring and Assessment Program (Oregon DEQ 2006).
The first step in the 2006 EPA SABS framework is to review current designated uses and criteria for a set of waterbodies. Next the researcher would describe SABS effects on the waterbodies’ designated uses. They would then select specific SABS and response indicators and define potential ranges in value of the SABS and response indicators. Last they would identify a response indicator value that protects the designated use and analyze any response associations (Paul et al. 2008). Having this guiding framework is very useful for managers who need to attain knowledge on SABS, in that they don’t have to come up with individual game plans every time around.
Using this framework, Bryce et al. 2008 and Bryce et al. 2010 were able to quantify criteria for protecting sediment sensitive aquatic species in the western United States. Bryce et al 2010 concluded minimum-effect bedded surficial sediment levels for aquatic vertebrates (fish and amphibians) were 5% and 13%, respectively for % fines (less than 0.06mm) and % sand and fines (less than 2 mm). They also concluded for macroinvertebrates the minimum-effect levels were smaller (3% and 10%) for the two particle size ranges. The process for determining these values can be seen in Figure 1 and Table 1. Similarly, Cormier et al. 2008 show in a hypothetical example that criteria could be set up where 95% of EPT in mid-Atlantic highland streams would be protected when levels remained below 7% bedded sediment fines. With numbers like these, managers are given a goal to seek to achieve for a given stream. Perhaps, if managers are dealing with a target predatory fish in a given stream, they will know to aim for the macroinvertebrate minimum-effect levels to insure food abundance and richness for their targeted fish species. If macroinvertebrate abundance is discovered not to be important, but a fish species is still sensitive to excess sediment, they will still have the marginal target minimum-effect threshold to attain and maintain for the species.
Figure 1. Quantile regression at the 90th-quantile of aquatic vertebrate index of biotic integrity (IBI) scores on areal percent fine sediments for (A) mountain streams of the western USA and (B) Oregon and Washington Coast Range streams. Similarity beween the two regressions is indicated by the overlapping 95% confidence intervals (CIs) for slope coefficients. (Figure from Bryce et al. 2008)
Table 1. Areal percent fine sediments (particles less than or equal to 0.06mm), aquatic vertebrate index of biotic integrity (IBI) score, and percent change in IBI in response to increases in areal percent fines for stream reaches in the Mountains ecoregion of the western USA. The reference value (5% fines) represents the 75th percentile of the fines distribution for the 169 least disturbed sites in the region. Values of IBI were calculated from the 90th-quantile regression equation (see Figure 1). (Figure from Bryce et al. 2008)
Areal fines (%) | IBI | Change in IBI (%) |
0 | 86.5 | 1.8 |
5 | 85 | Reference |
10 | 82.5 | -2.9 |
16 | 80 | -5.9 |
20 | 78.5 | -7.6 |
30 | 74.5 | -12.4 |
40 | 70.5 | -17.1 |
50 | 66.5 | -21.8 |
60 | 62.5 | -26.5 |
70 | 58.5 | -31.2 |
80 | 54.5 | -35.9 |
90 | 50.5 | -40.6 |
Decreases in abundances of animals and increases in bedded sediments seem to be themes repeated throughout our country. Unfortunately, if you can’t somehow quantify how much the substrate has been altered, you won’t be able to tell how bad it is or isn’t. Having a quantifiable number like the one the SABS framework can supply is an important first step. With this in hand, managers can be sure of where they are and where they want to get. They can focus on the causes and figure out ways to decrease sedimentation, erosion and runoff. By monitoring the streams they have done this work to, they can then know if they are reaching their goal and providing the help certain species need. Bryce et al. 2008 do point out that managers and stakeholders may support less-stringent criteria for budgetary or political reasons, but they will be less protective of aquatic life potential.
An example where sediment criteria might prove useful is in the upper Tennessee River watershed. Being a hotspot for both fish and freshwater mollusks, pollutants of any kind could have far reaching detrimental effects on species richness and abundance. If it could be shown that high percentages of SABS was correlated to low fish and mussel species richness in reaches of stream in this region, managers would likely aim to lessen sediment loads throughout the watershed. In order to do this, they would need a benchmark number to focus their efforts on, and this type of number might only be achievable through a framework like the EPA has developed. By having a number established as an acceptable level, managers could identify reaches that were below the standard and figure out ways to bring them up to par. Also, managers might be monitoring reaches that were well below the criteria, and could use the number as a threshold they would wish to stay away from for the sake of the animals.
The next step needs to be unifying these criteria among states and agencies. It isn’t helpful for states with similar streams and similar species assemblages to have completely different ambient water quality criteria for sediments. Turbidity in one state can’t be compared to percent bedded sediments in another, to draw any reasonable conclusions. Also, turbidity could be caused by one type of sediment or runoff in one stream while being caused by a completely different type of sediment or runoff in another. So, it might prove difficult to associate factors dealing with turbidity among different streams.
Conclusions
The SABS framework of the EPA seems to be a step in the right direction for developing ambient water quality criteria for the United States. There are two other promising methods being developed. One is field-derived species-sensitivity distributions. These are distributions patterned after the methods developed for criteria development based on toxicity tests. In order to develop these distributions, effects levels for several species that live in and are exposed to conditions in a class of ecosystem are identified by using quantile regression models of field data (Paul et al. 2008). The other is specific tolerance values. This method models the relationship between an exposure measure and a taxon. After tolerance values are established for a group of organisms, the conditions of new sites can be assessed on the basis of whether taxa from tolerant of sensitive groups are predominantly collected (Yuan 2006). Regardless of which method is used, national criteria need to be created to help managers.
Future efforts could be undertaken in stream subclasses to refine criteria development efforts, specific to a given state, ecoregion, geological type, stream slope, stream power class, or taxon of interest (Bryce et al. 2008). Results indicate that IBI’s (indexes for biotic integrity) calculated for same stream type, at different regional scales and sample sizes, yielded very similar prediction for the potential response of IBI to fine sediments (Bryce et al. 2010). This could be immensely beneficial to stream habitat managers. It would help them quickly identify the problem and get them set on a path for a fix. It would also help in that when managers were presented with the job of tackling a sediment issue in a stream, they could compare their project with like projects in the same region.
The potential seems to be there for national criteria for SABS levels in streams being available to managers in the future. Even if national standards are not quantified, researchers should be able to attain ambient water quality criteria for sediment in local streams, by following the EPA SABS framework themselves. By helping the streams rid themselves of the sediment, the streams will likely respond with creating new habitats for extirpated wildlife to inhabit. This could help solve issues with connectivity in streams as more like habitats once again stretch longer distances through the stream. It could also provide more habitats for species whose habitats are very specific and rare due to sedimentation. Personally, I have observed stretches of the Clinch River in Virginia and Tennessee where this would apply in regards to freshwater mussels. Many of the endangered species of mussels residing in this bio hotspot require fast-moving, highly oxygenated riffles for survival. Due to urbanization in the upper reaches of the watershed, many riffles have been inundated with silt fines. Many other factors could have lead to the decreases in abundance and diversity of mussels in these areas, but it seems apparent that sediment has something to do with it. In lower reaches of the same river, the lack of human impacts have allowed individual shoals to hold as many as 50 species of freshwater mussels. The riffles in these areas are full of larger particles and loose sands and silt fines are only found where emergent vegetation has helped it to settle.
Legislature will need to be reviewed throughout the country and world for that matter regarding sediment as a pollutant and how it should be quantified and reduced. Further research will hopefully convince authorities to revise current standards. As part of the Clean Water Act, listed streams should have sediment criteria developed to go along with all of the other various water quality thresholds trying to be maintained. It seems that many like to get on bandwagons for issue such as this, but few are willing to be the first in line. If just one state or region would adopt more stringent criteria involving sediments, than perhaps many other states or the entire nation would follow.
Through research, it is noticeable that there is very little study being done outside the mountainous northwest of the United States on this issue. Future studies should be done to figure out if the same rules would apply to ecosystems across the nation. Species diversity is much higher in the Mississippi basin and could produce much different results from what researchers in mountainous salmonid streams might find.
We have the opportunity in this country to relay our findings to the developing world. We have already caused many of the excess sediment problems to our nation’s rivers from large-scale agriculture and developing land around streams. In nations where this has yet to occur, our findings could help them plan better than we did, as to preserving their own aquatic faunas. For example, something as easy as preserving riparian zones during land development could have significant positive effects on biotic and abiotic conditions in flowing waters (Henley et al. 2000). If sediment is one of the biggest pollutants to streams in our country it will be in others, so efforts should be made to prevent this from happening elsewhere.
In conclusion, we need national ambient water quality criteria for sediments in streams. If these criteria have been put in place for many other water quality parameters, there is no reason why we can’t develop them for sediments. Even region-by-region criteria would be better than what we have currently.
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