2.5. Turbidity


This section is authored by Dr. Radha Pyati, Dean of the College of the Sciences and Mathematics at West Chester University of Pennsylvania.


2.5.1. Description and Significance

In its natural state, the St. Johns River, like other blackwater rivers, swamps, and sloughs, has a high concentration of colored dissolved organic material (CDOM) that stains the water a dark brown color. The natural decay of plant materials stain the water to appear somewhat like tea in color. The St. Johns River, in particular, has a varied mix of dark-stained water from rainwater flow through the slow moving backwaters, and nearly clear contributions from large springs such as Blue Spring, De Leon Springs, Silver Springs (through the Ocklawaha River), and others. Heavy rains flush tannin-stained waters out of the slow-moving sloughs, swamps, and backwaters and into the tributaries and mainstem of the LSJR. Color and turbidity are different properties of water, and both may arise from natural and anthropogenic sources. Turbidity is a reflection of how cloudy a water body appears, unlike the light absorption properties described by color. Turbidity is described on the Florida DEP website as:

Turbidity is a measure of the suspended particles in water. Several types of material cause water turbidity, these include: silt or soil particles, tiny floating organisms, and fragments of dead plants. Human activities can be the cause of turbidity as well. Runoff from farm fields, stormwater from construction sites and urban areas, shoreline erosion and heavy boat traffic all contribute to high levels of turbidity in natural waters. These high levels can greatly diminish the health and productivity of estuarine ecosystems (DEP 2009f).

Three types of particles optically scatter light in the water column: suspended solids, particles of bacterial and algal origin, and micron-sized particles of CDOM. All are present in the dominantly freshwater portion of the LSJR (Gallegos 2005); however, the turbidity is dominated by both phytoplankton (mostly single-cell plants) and suspended solids from human impact (most often sediment or industrial waste) called non-algal particulates (NAP). NAP comes from such activities as sediment erosion from construction, land clearing and timber harvesting sites; stormwater runoff in urban and industrial areas, dredging, and solids from industrial outfalls (Gallegos 2005). During heavy rains, these sources may input a large volume of NAP into tributaries of the river. To address this, Florida has an extensive storm-water permitting program to limit stormwater impact. As discussed above, stormwater and drainage systems once considered non-point sources are now registered and permitted under the National Pollutant Discharge Elimination Program (NPDES) (DEP 2009e).

In contrast to turbidity in freshwater, in more haline (salty) portions of the LSJR, scattering of light is dominantly from materials that are of larger size, such as sediment (Gallegos 2005).

Periods of drought and rainfall can significantly affect turbidity. During periods of drought, flow from the tannin-stained backwaters decreases dramatically, but the flow from the clear springs diminishes less. When this happens, the water may become significantly clearer, and optical absorption by CDOM diminishes to below normal levels. With decreased CDOM and higher light penetration, phytoplankton are able to use the high nutrient concentrations more efficiently and readily undergo accelerated growth (Phlips et al. 2007). In rainy periods after a drought, the St. Johns River may actually become more darkly stained from CDOM than usual, as rainfall moves the stalled and tannin-stained waters into the mainstem of the LSJR again. Under these conditions, CDOM absorption is the most influential optical property in a blackwater system such as the LSJR (Phlips et al. 2000). In other events, and at specific locations and times, phytoplankton or NAP will dominate light loss in the water column and can be assessed by comparing turbidity levels with chlorophyll-a levels, which indicate algal content.

Turbidity levels in tributaries can increase during periods of drought under certain conditions, such as near constant industrial and WWTF output, algal blooms, or, more commonly after episodic rain events. For instance, sediment from construction, land clearing and timber harvesting sites, coupled with stormwater runoff, can be washed into the adjacent waters and overwhelm the other components. It is not difficult to spot sediment-laden water due to its appearance, often having a resemblance to “coffee with cream,” as shown in Figure 2.25 for example.

Figure 2.22
Figure 2.25 Turbid water from McCoy Creek entering the LSJR on 17 July 2008. Courtesy of Christopher Ball.

Turbidity (algal and sediment particulate) and color are the two primary light attenuating factors in the LSJR that prevent light from reaching rooted submerged plants and thus hinder aquatic photosynthesis. Small plants and plantlike bacteria have evolved to float or suspend themselves in the upper levels of the water column to remain in the sunlight. At high concentrations, their combined scattering may not pass sufficient light to large plants attached to the bottom, like the river grasses that feed and serve as nursery habitat for juvenile fish and shrimp. Submerged aquatic vegetation (SAV) can suffer from a lack of light resulting from high turbidity and from sediment cover, from shading by smaller plants coating their leaf surfaces, or masking by floating algae. This has a large impact on animals, which depend on the grasses for food and shelter.

Algal blooms can dominate turbidity when excess nutrient and sufficient background algal concentrations combine to produce prolific growth of the algal biomass. In this situation, the planktonic or filamentous algae can reduce visible depth, affecting the rooted submerged aquatic vegetation. This is referred to as a hypereutrophic condition. A good discussion of trophic state is found on the website of the Institute of Food and Agricultural Sciences at the University of Florida (IFAS 2009). While high trophic state index (TSI) values indicate high primary (plant) productivity, often that is part of an unbalanced ecosystem with very high nutrient and a large algal biomass that has large fluctuations in dissolved oxygen. A reduction in water clarity due to algal blooms is distinguishable from sediment turbidity by measurement of total chlorophyll-a at a level greater than 40 µg/L (SCCF 2014). This is not an optimum, healthy state for the entire ecosystem of the water body.

Figure 2.26 shows turbidity values in the LSJR since 1997. The box indicates the median ± 25% of the data points (middle 50%). In several years, the highest value recorded was significantly higher than the interquartile range described by the green box; for those years, the high value is higher than the maximum value on the graph. A background turbidity level in the LSJR varies from single digit values to 12-15 Nephelometric Turbidity Units (NTUs) along the mainstem (Armingeon 2008), and anything over 29 NTUs above background is considered to exceed Florida state standards FAC 62-302.530 (62-302 F.A.C.; DEP 2013i). While the state criterion for turbidity is 29 NTU above background, background levels vary in the LSJRB; therefore, 29 NTU has been used as the threshold in the graphs.

Figure 2.26 Yearly turbidity in the Lower St. Johns River Basin; 1997-2019. Data are presented as a box-and-whiskers plot with the green boxes indicating the median value ± 25% (middle 50% of data) and the blue whiskers indicating the minimum and maximum values in the data set.
Figure 2.26 Yearly turbidity in the Lower St. Johns River Basin; 1997-2019.
Data are presented as a box-and-whiskers plot with the green boxes indicating the median value ± 25% (middle 50% of data)
and the blue whiskers indicating the minimum and maximum values in the data set.

Over this period, there have been changes in measurement techniques, spatial sampling changes, and many other factors, but clearly since 1997, the median values of turbidity in the LSJR are below the acceptable limit.

Long-term trends have generally shown stable turbidity levels over decades, but a look at recent years reveals recent changes. First, the single waterbody in the Lower St. Johns River Basin with the highest turbidity values over 2017-19 is Sherman Creek (WBID 2227), a small waterbody of about 0.086 square miles, located just south of the mouth of the St. Johns River and containing much of Atlantic Beach, Florida. Figure 2.27 shows turbidity values in the waterbody, and the median values of all three years fall below the WQC of 29 NTU above background. However, 2018 and 2019 seems to have been a year of elevated turbidity events in Sherman Creek; notably, in 2019, a portion of the third quartile of data lies above the WQC.

Figure 2.27 Yearly turbidity in Sherman Creek, WBID 2227; 2017-2019. Data are presented as a box-and-whiskers plot with the green boxes indicating the median value ± 25% (middle 50% of data) and the blue whiskers indicating the minimum and maximum values in the data set.
Figure 2.27 Yearly turbidity in Sherman Creek, WBID 2227; 2017-2019. Data are presented as a box-and-whiskers plot with the green boxes indicating the median value ± 25% (middle 50% of data) and the blue whiskers indicating the minimum and maximum values in the data set.

As well, turbidity can be affected by sediment from construction projects, such as the Jacksonville Harbor Deepening project, which began in February 2018 and will encompass a thirteen-mile stretch of the mainstem of the river beginning at the river’s mouth at the Atlantic Ocean (ACOE 2020b; ACOE 2020a; JAXPORT 2020b). The project lies within three waterbodies along the river’s mainstem: WBIDs 2213A, 2213B, and 2213C. Turbidity levels in 2017-19 for those three waterbodies and the next upstream waterbody, WBID 2213D, are shown below. Observations fall below the WQC, with the only exception occurring in WBID 2213D, upstream of the project.

Figure 2.28 Turbidity measurements in waterbodies along the mainstem of the Lower St. Johns River Basin (WBIDs 2213A, 2213B, 2213C, and 2213D). Markers indicate individual observations.
Figure 2.28 Turbidity measurements in waterbodies along the mainstem of the Lower St. Johns River Basin (WBIDs 2213A, 2213B, 2213C, and 2213D). Markers indicate individual observations.

2.5.2. Data Sources

The primary source for this evaluation is the FDEP’s Watershed Information Network (WIN) database, the Florida STORET database and the EPA-mandated reports required by the CWA, such as the Florida 303(d) report of impaired waters. These reports become the basis for future water quality management and restoration efforts. These are publicly available online at DEP 2004 and DEP 2009d. Previous versions of this report used EPA STORET data.

2.5.3. Limitations

In 1998, under the Florida standards (62-303 F.A.C.; DEP 2013i), 16 waterbodies in the LSRJB were listed as impaired for turbidity. Many of these were urban streams between the city of Jacksonville and Mayport, areas where urban runoff may have been a problem. Many have since been “delisted” in the CWA process. This may truly indicate substantial improvements, but it may also have been partly a function of the sampling timing during pre-hurricane drought conditions in 2004, which greatly reduced runoff and associated turbidity. For example: the earlier 303(d) report listed Cedar River and Goodbys Creek, as well as the mainstem of the river above the Dames Point area, at high risk of turbidity impairment, while later assessments, based on sampling in 2004, did not find turbidity impairments. Additionally, we have chosen to use virtually all the STORET data in spite of changes in methodology, uneven spatial and temporal sampling, and other issues that limit both the validity and generalization of the trend.

2.5.4. Current Conditions

STATUS: Satisfactory
TREND: Unchanged

Based on current data available from WIN and STORET, turbidity conditions seem to be satisfactory for the LSJRB, as seen in the first figure above. In 2019, the highest turbidity value observed was 70 NTU. Year to year, these values vary due to rainfall events, land-disturbing activities, and other such occurrences. As a result, the STATUS of turbidity is Satisfactory, and the TREND is Unchanged.

In November 2019, fifteen waterbodies were included in the final list of waterbodies delisted from the 1998 Florida 303(d) list, for reasons of non-impairment and movement onto the planning list.

2.5.5. Future Outlook

Current management of turbidity in Duval County, for example, includes a requirement for land-disturbing activities to be overseen by a developer’s certified staff, routine visits of land-disturbing sites, review of erosion control plans, and a citizen reporting mechanism. Heightened public awareness and improved engineering sediment control practices are bringing improvements in this area. Finable events over the past few years and the press they received will help keep the pressure on proper engineering practices. Vigilance in design of retention and detention ponds, sediment fences and public monitoring all can help. Reporting of turbidity events and sediment discharges near land-clearing and construction projects, particularly future Developments of Regional Impact (DRI) and monitoring existing municipal separate storm sewer system (MS4) areas for storm runoff should help ensure the best outcomes for the LSJR. Tributaries are particularly prone to turbidity events after a heavy rainfall.

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