Pollutant Loading Sources: Water and Nutrient Budgets

Pollutant Loading Sources: Water and Nutrient Budgets

As mentioned earlier, several investigators have prepared water and nutrient budgets for Lake Tarpon (CCI 1990; N.S. Nettles & Associates, Inc. 1991; KEA 1992 and Robison 1994). PBS&J (1998) re-evaluated the pollutant loading sources based on the most current data and developed revised water and nutrient budgets. The water and nutrient budgets shown in Tables A-1, A-2, and A-3 account for the water and nutrient flux from groundwater developed by ERM (1998).

Table A-1. Lake Tarpon annual water budget.

INFLOWS

CUBIC FEET/SECOND

PERCENT OF TOTAL

Direct Runoff (modeled)

20.8

42.2

Precipitation

16.5

33.5

Brooker Creek (gaged)

9.1

18.5

Septic Tanks

0.1

0.2

Surficial Aquifer Seepage

1.9

3.8

Floridan Aquifer Seepage

0.9

1.8

TOTALS

49.3

100

OUTFLOWS

CUBIC FEET/SECOND

PERCENT OF TOTAL

Outfall Canal Discharge*

33.7

68.4

Evapotranspiration

15.6

31.6

TOTALS

49.3

100

* Revised as the difference between total inflow and the evapotranspiration outflow due to the unreliability of reported discharge volumes through the Lake Tarpon Outfall Structure.

Table A-2. Lake Tarpon annual total nitrogen budget.

INFLOWS

TONS/YEAR

PERCENT OF TOTAL

Direct Runoff (modeled)

27.45

48.6

Atmospheric Deposition

9.99

17.7

Brooker Creek (gaged)

10.45

18.5

Septic Tanks

6.49

11.5

Surficial Aquifer Seepage

1.78

3.1

Floridan Aquifer Seepage

0.35

0.6

TOTALS

56.51

100

OUTFLOWS

TONS/YEAR

PERCENT OF TOTAL

Outfall Canal Discharge

35.17

62.2

Fish Harvest

0.70

1.3

Sedimentation/Macrophyte Biomass*

20.64

36.5

TOTALS

56.51

100

* Calculated as the difference between total inflow and the sum of the outfall canal discharge and fish harvest outflows.

Table A-3. Lake Tarpon annual total phosphorus budget.

INFLOWS

TONS/YEAR

PERCENT OF TOTAL

Direct Runoff (modeled)

4.03

68.8

Atmospheric Deposition

0.20

3.4

Brooker Creek (gaged)

0.61

10.4

Septic Tanks

0.82

14.0

Surficial Aquifer Seepage

0.13

2.2

Floridan Aquifer Seepage

0.07

1.2

TOTALS

5.86

100

OUTFLOWS

TONS/YEAR

PERCENT OF TOTAL

Outfall Canal

1.39

23.7

Fish Harvest

0.24

4.1

Sedimentation/Macrophyte Biomass*

4.23

72.2

TOTALS

5.86

100

* Calculated as the difference between total inflow and the sum of the outfall canal discharge and fish harvest outflows.

Based on the work done to develop the water and nutrient budgets, PBS&J (1998) made the following observations and conclusions.

· The Lake Tarpon watershed consists of three major drainage basins including the Brooker Creek basin, the South Creek basin, and the Lake Tarpon basin. Direct runoff from the Lake Tarpon drainage basin (42.2 percent), and precipitation on the lake surface (33.5 percent), account for a total of 75.7 percent of the freshwater inflows to the lake. The gaged Brooker Creek flow (18.5 percent) is also a significant source of freshwater inflow. Hydrologic inflows from the surficial aquifer (3.8 percent), the Floridan aquifer (1.8 percent), and septic tanks (0.2 percent) are relatively insignificant.

· Hydrologic outflows from Lake Tarpon are limited to outfall canal discharges (68.4 percent) and evapotranspiration (31.6 percent).

· Compared to the South Creek basin and both the gaged and ungaged portions of the Brooker Creek basin, the Lake Tarpon basin is by far the most significant contributor to anthropogenic TN and TP loadings to the lake. Although the gaged Brooker Creek basin is a very significant source of hydrologic inflows to the lake, these findings strongly suggest that external pollutant load reduction strategies implemented in the Lake Tarpon basin, as opposed to the South Creek or Brooker Creek basins, will be most effective in trophic state management of Lake Tarpon.

· Of the six identified sources of nutrient inflows to Lake Tarpon, only direct runoff and septic tanks are considered to be manageable sources that could be reduced through the implementation of stormwater best management practices (BMPs) and construction of central sewer facilities, respectively.

· Approximately 48.6 percent of the TN load and 68.8 percent of the TP load to Lake Tarpon are contributed from direct stormwater runoff from the Lake Tarpon basin.

· Based upon the sub-basin ranking and prioritization procedure used in the pollutant loading analysis, four manageable hydrologic units (MHUs = combinations of hydrologically connected sub-basins), and two individual sub-basins, have been selected for the potential implementation of non-point source BMPs. These basins are shown in Figure A-8 and listed in order of decreasing priority below:

- Group-B MHUs (contributing sub-basins 49, 51, 52, and 54);

- Group-D MHUs (contributing sub-basins 5 and 6);

- Group-A MHUs (contributing sub-basins 60, 62, 63, 65, and 66);

- Group-C MHUs (contributing sub-basins 45, 46, and 47);

- Individual sub-basin 23; and

- Individual sub-basin 21.

The cumulative nutrient load from the four priority MHUs and two individual sub-basins constitutes 6.32 and 0.73 tons of TN and TP per year, respectively. This represents 11.2 percent of the total annual TN load, and 12.5 percent of the total annual TP load, from external sources. In addition, this represents 23.0 percent of the annual TN, and 18.1 percent of the annual TP load, from direct runoff, respectively.

· Approximately 11.5 percent of the TN load and 14.0 percent of the TP load to Lake Tarpon are contributed from septic tank seepage in the Lake Tarpon basin.

· A total of 1,076 residences with septic tanks occur within the Lake Tarpon basin. Sub-basins 5, 6, 7, 9 and 13 generate the highest modeled septic nutrient loads. These sub-basins correspond to predominantly residential areas where no central sewer service, or only partial service, is available. Priority should be given to removal of septic systems and the extension of central sewer service in these sub-basins.

· Based on modeling results, the provision of central sewer to all residences with septic systems would result in an annual load reduction of 6.37 tons for TN and 0.78 tons for TP. This corresponds to 11.3 and 11.9 percent of the total annual TN and TP loads, respectively.

· Septic systems are regulated in an effort to minimize the potential for groundwater and surface water contamination. However, site specific conditions (such as high water table or improper soils) or lack of proper maintenance of the system may lead to the reduced effectiveness of treatment and eventual total failure of the septic systems which may contribute to locally significant groundwater and surface water pollutant loadings. The combined external load reduction strategies of providing enhanced stormwater treatment of runoff from the priority MHUs and sub-basins, and central sewer to all remaining residences with septic tanks, would result in a 15.7 and 24.6 percent reduction in total annual TN and TP loads, respectively.

· The Lake Tarpon basin is not homogeneous with regard to its physical and developmental characteristics. Anthropogenic loadings of TN from non-point sources, point sources (e.g., effluent reuse) and septic tanks are all higher from the west lake region than from the east lake region. With regard to TP, the sum of these three anthropogenic loading sources is also higher for the west lake region. The west lake region and the northeast quadrant of the lake generally represent the zones of highest pollutant loading.

· Seepage from the surficial aquifer accounts for approximately 3.1 and 2.2 percent of the total annual TN and TP loads to the lake, respectively. Nutrient concentrations in the surficial aquifer are affected by land application of fertilizers and spray irrigation of reclaimed water, as well as natural processes.

· Spray irrigation sites in Anderson Park (sub-basins 10, 11 and 12), Highland Lakes (sub-basin 24) and Lansbrook (sub-basin 53) account for virtually all of the modeled loads from effluent land application. Nutrient loads to the lake from effluent land application are potentially measurable with regard to TN loadings, primarily in the form of nitrate. Due to the different reactive processes and fate of phosphorus in the subsurface environment, TP loadings from effluent land application are calculated to be close to zero.

· With regard to management considerations, effective assimilation of nutrients from spray irrigation is extremely dependent upon effluent application rates and the concurrent antecedent conditions of the applicable soils. When applied to common areas (e.g., medians, public parks, etc.) under a managed rate control program, nutrient loadings to the lake from effluent land application can be effectively minimized. If, however, reclaimed water is made available to large residential areas in the Lake Tarpon basin, especially those on the west side of the lake where the soils are well-drained, the potential for over-application will likely increase. On a cumulative basis, unmanaged effluent land application in the Lake Tarpon basin has the potential to become a measurable component of the overall TN load to the lake.

· Atmospheric deposition accounts for approximately 17.7 percent of the TN loadings, and 3.4 percent of the TP, loadings to Lake Tarpon. Because of the extremely diffuse nature of air pollutants, relatively little can be done in terms of specific management actions within a local watershed to reduce atmospheric deposition to a target waterbody.

· While discharges from the gaged Brooker Creek basin also constitute a significant source of TN loadings to the lake (18.5 percent of the total annual TN load), viable load reduction strategies probably don’t exist given the relatively natural character of the Brooker Creek watershed and its status as a County preservation area.

· Given the large contributions of the relatively unmanageable sources of atmospheric deposition and Brooker Creek to the overall TN load, and the fact that the lake is close to being phosphorus limited based on the in-lake TN:TP ratio, external pollutant load reduction strategies for Lake Tarpon would likely be more effective if an emphasis was placed on phosphorus controls rather than nitrogen controls.

· The annual nutrient budgets for Lake Tarpon indicate that approximately 36.5 percent of the TN load, and 72.2 percent of the TP load, are retained within the lake via both deposition in lake sediments and assimilation in macrophytic plant tissue. Although it is difficult to accurately quantify the mass of nutrients annually released back into the water column in association with macrophyte senescence and decomposition, water quality trends indicate that this mass may be very substantial following large scale chemical treatment of hydrilla. These internal nutrient stores represent a potentially major source of nutrient loadings under certain conditions via internal recycling. Measures to reduce internal recycling should be pursued as a means of reducing the lake trophic state index.

Monitoring of the nutrient budget for Lake Tarpon is an important tool in determining the effectiveness of implemented management strategies and in monitoring changes in the lake trophic condition as a response to changes in the watershed and in-lake processes. Pinellas County monitors water quality in Lake Tarpon monthly and they have collected water quality and quantity data for the inflows to the lake. To date, outflows to the lake have been estimated based on generally accepted practices. However, direct measurement of outflow through the Lake Tarpon Outfall Structure would aid in the refinement of the Lake Tarpon nutrient budget and in evaluating the success of the implemented management strategies. The District, Pinellas County and the United States Geological Survey began working together in 1999 to collect outflow data at the Lake Tarpon Outfall Structure.

Pollutant Load Reduction Strategies

Historical data for Lake Tarpon indicated that the annual average TSI was about 50 and during development of the 1994 SWIM Plan, Lake Tarpon had experienced annual average TSI values around 54. Given the inherent variability in the index, the TSI value may not have deviated substantially from the historic TSI. This, coupled with the need to allow Pinellas County to develop a plan to evaluate non-point source reductions on a cost/benefit basis, lead the District to set an interim PLRG of zero.

Since completion of the 1994 Lake Tarpon SWIM Plan, the annual average TSI value for the lake has increased and for the period from May 1996 to April 1997 PBS&J (1998) calculated an annual average TSI of 59. This increasing productivity as measured by the amount of algae (chlorophyll-a) in the water results from an increase in nutrients entering the lake and from the recycling of these nutrients once they have entered the lake. Therefore, PBS&J (1998) evaluated various management strategies to control external and internal sources of nutrients to Lake Tarpon and the discussion below is based on their evaluation.

Control of External Nutrient Sources

As indicated in Tables A-2 and A-3, external nutrient loading sources to Lake Tarpon include atmospheric deposition, direct runoff (modeled), Brooker Creek, septic tanks, and seepage from the surficial and Floridan aquifers. The DBMP (PBS&J, 1998) concluded that of these external sources, the only manageable sources (e.g., can feasiblely be reduced through remediative measures) were direct runoff (48.6 percent of the total TN load and 68.8 percent of the total TP load) and leachate from septic tanks (11.5 percent of the total TN load and 14.0 percent of the total TP load). The other major external sources, including atmospheric deposition and groundwater inflows, are considered to be unmanageable from a practical standpoint. In addition, nutrient loadings from Brooker Creek were also considered to be essentially unmanageable given the relatively natural character of the basin. Furthermore, loadings from the Brooker Creek watershed in Pinellas County are not likely to be reduced through the construction of regional stormwater treatment facilities due to the fact that Pinellas County has already purchased the majority of the contributing land area as a preservation area, and such facilities would likely be inconsistent with the designated uses of the Preserve. (Since PBS&J was under contract to Pinellas County, they did not consider the Brooker Creek watershed in Hillsborough County. The District has begun working with Hillsborough County to investigate opportunities for water quality improvement and habitat and hydrologic restoration in the Brooker Creek watershed in Hillsborough County.)

With the exception of sediment removal, the most costly lake management options typically involve the rehabilitation of stormwater and wastewater discharges as a means of reducing external nutrient loadings. Given the relative importance of external nutrient loads to the Lake Tarpon nutrient budget (compared to internal loads from nutrient recycling), and the potentially high cost of the various external load reduction strategies, only those management strategies aimed at external load reduction were subjected to cost-effectiveness analyses by PBS&J (1998). The results of those analyses are discussed in the following sections.

Stormwater Retrofit of Priority Sub-basins - During development of the nutrient budgets for Lake Tarpon, PBS&J (1998) delineated sub-basins within the Lake Tarpon watershed in Pinellas County (Figure A-8). Modeling techniques were used to estimate freshwater inflows and pollutant loadings to Lake Tarpon and to prioritize sub-basins for implementation of BMPs (Coastal 1995). Non-point source loadings for 67 sub-basins in the Lake Tarpon drainage basin were estimated using an empirical hydrologic model based on land use, soils, rainfall, and sub-basin boundaries. Hydrologically connected sub-basins were treated as a single manageable unit, and were termed “manageable hydrologic units” (MHUs). The MHUs and/or individual sub-basins with the highest TN, TP, and TSS loadings from direct runoff were identified and then ranked for priority based on pollutant load and other logistical factors. Table A-4 shows the area, modeled annual flows and TN and TP loads for the MHUs and individual sub-basins in priority order.

Table A-4. Summary of modeled loads from the priority MHUs and individual sub-basins.

Treatment Area

Area

(acres)

Runoff

(cfs)

(tons/year)

Group B MHUs (sub-basins 49, 51, 52, 54)

713.3

1.29

1.63

0.22

Group D MHUs (sub-basins 5, 6)

436.2

1.03

1.61

0.15

Group A MHUs (sub-basins 60, 62, 63, 65, 66)

569.8

0.80

1.11

0.20

Group C MHUs (sub-basins 45, 46, 47)

337.2

0.68

0.85

0.08

Sub-basin 23

211.6

0.44

0.67

0.05

Sub-basin 21

114.6

0.24

0.45

0.03

Totals

% of direct runoff load

% of total loads

2,382.7

---

4.48

21.5%

9.1%

6.32

23.0%

11.2%

0.73

18.1%

12.5%

PBS&J (1998) evaluated cost effectiveness of retrofitting the four priority MHUs and the two priority individual sub-basins using wet detention stormwater ponds and alum injection stormwater treatment ponds. This analysis is summarized below.

Wet Detention Ponds - The amount of TN and TP load reduction that may be accomplished through the use of wet detention ponds was estimated by completing a conceptual design of ponds necessary to treat the regulatory runoff volume per District Management and Storage of Surface Waters (MSSW) standards. The following assumptions were made to estimate the amount of TN and TP load that would be available for treatment, and the load reduction that could be accomplished through use of wet detention ponds:

· 90 percent of all storms are of one inch rainfall or less.

· 75 percent of all those storms are temporally spaced to allow bleeddown of the ponds, so that the full storage volume is available for a new storm.

· TN treatment efficiency is 0.30, and TP treatment efficiency is 0.60.

Based on this analysis, the annual non-point source nutrient loads from the priority MHUs and individual sub-basins can feasiblely be reduced by 20.3 percent for TN, and 41.1 percent for TP, using wet detention ponds. Total costs, including land acquisition, construction, and operation and maintenance were estimated to be $2,309,622 for the 20-year life span of six (6) wet detention ponds, or approximately $384,937 per pond. The total TN reduction was estimated to be 1.28 tons or 2,560 lb/year (51,200 lb in 20 years) and the TP reduction was estimated to be 0.30 tons or 600 lb/year (12,000 lb in 20 years). Thus, the unit cost of treating direct runoff from the priority MHUs and individual sub-basins with wet detention ponds is ($2,309,622/51,200 lb TN), or $45/lb TN, and ($2,309,622/12,000 lb TP) or $192/lb TP.

Wet Detention Ponds Enhanced with Alum Injection - The amount of TN and TP load reduction that may be accomplished through the use of alum injection with sediment traps was estimated by completing a conceptual design of the systems necessary to treat runoff from the five priority MHUs. Design criteria were based on specifications for other local alum systems that have recently been designed and constructed (ERD, 1994). The assumptions made to estimate the amount of TN and TP load that would be available for treatment were the same as those made for wet detention ponds, with the exception of the load reduction. The load reductions associated with alum injection systems were:

· TN removal efficiency for injected alum is 0.40, and TP treatment efficiency is 0.90.

Based on this analysis, the annual non-point source nutrient loads from the priority MHUs and individual sub-basins can feasiblely be reduced by 27.0 percent for TN and 61.1 percent for TP using alum injection ponds. The total cost of constructing, operating and maintaining six alum injection treatment facilities with sediment traps over the 20-year life of the project was estimated to be $4,136,188. The total TN reduction was estimated to be 1.71 tons or 3,420 lb/year (68,400 lb in 20 years) and the TP reduction was estimated to be 0.44 tons or 880 lb/year (17,600 lb in 20 years). Thus, the unit cost of treating direct runoff from the priority MHUs and individual sub-basins with wet detention ponds is ($4,136,188/68,400 lb TN), or $60/lb TN, and ($4,136,188/17,600 lb TP) or $235/lb TP.

Conversion of Septic Tanks to Central Sewer - The DBMP identified 1,076 septic tanks in the Lake Tarpon drainage basin and estimated that approximately 0.20 million gallons per day (mgd) of leachate was cumulatively discharged from these systems (PBS&J 1998). This resulted in estimated TN and TP loadings of approximately 6.49 tons/year of TN and 0.82 tons/year for TP or about 11.5 and 14.0 percent of the total external TN and TP loads to the lake, respectively. Converting septic tank service areas to sanitary sewer service areas could potentially reduce the TN load to Lake Tarpon by approximately 6.37 tons/year, or 12,740 lb/year (98 percent load reduction), and reduce the TP load to the lake by approximately 0.78 tons/year, or 1,560 lb/year (95 percent load reduction). This analysis did consider the change in nutrient loadings to the lake as a result of increased effluent disposal.

The total cost for replacing septic tank systems with sanitary sewer was estimated to be $9,264,400. The total TN reduction was estimated to be 12,740 lb/year (254,800 lb in 20 years) and the TP reduction was estimated to be 1,560 lb/year (31,200 lb in 20 years). Therefore, the unit cost of converting all septic tanks in the basin to sanitary sewer service is ($9,264,400/254,800 lb TN), or $35/lb TN, and ($9,264,400/31,200 lb TP), or $297/lb TP. (Note that this cost is amortized over 20-years.)

Summary of Cost Effectiveness of External Nutrient Removal Strategies

Table A-5 presents a summary of the TN and TP reduction potential of the two primary external load reduction alternatives, septic tank conversion and alum injection with sediment traps. This table also shows the unit cost (cost per pound of TN and TP removed) and total cost (project cost amortized over a 20-year facility life) for each alternative.

Table A-5. Summary of the TN and TP load reduction potential, unit costs and total costs of the two primary external load reduction alternatives.

BMP

Potential Load Reduction

(lbs/year)

Unit Cost of Load Reduction

($/lb)

Total Cost of Load Reduction*

($/year)

Septic Tank Conversion

12,740

1,560

297

$445,900

$463,320

Alum Injection with Sediment Traps

3,420

880

235

$205,200

$206,800

TOTALS

% total load reduction

16,160

14.3%

2,440

20.8%

---

$651,100

$670,120

* Annual costs amortized over 20-year facility life.

Control of Internal Nutrient Sources

Internal nutrient sources include sediment resuspension, movement of nutrients from the sediment into the overlying water and decomposition of organic matter. Control of internal nutrient loadings include sediment removal or inactivation of sediment phosphorus by alum treatment, dilution or flushing of nutrient rich water and mechanical harvesting of nuisance aquatic plants. Due to potential toxic effects of alum in estuarine waters downstream of Lake Tarpon, the use of whole lake alum treatments was not considered. Sediment removal was not considered due to the fact that lake sediments are relatively low in organic content and the lake is relatively deep. Based on modeling results, sediments act as a sink for phosphorus and only provide a small flux of nitrogen. For these reasons, only macrophyte harvesting and increased lake flushing and dilution through the implementation of an enhanced lake level fluctuation were considered by PBS&J (1998) as internal nutrient load reduction strategies.

Flushing and Dilution - Flushing and dilution are well-documented lake management techniques that involve increasing the rate at which the nutrient mass is flushed from the lake combined with the use of higher quality dilution water to reduce in-lake concentrations of nutrients. Flushing and dilution serve to reduce the concentration of nutrients, and the period of time that aquatic vegetation is exposed to these nutrients. The reduced nutrient concentrations should lead to reduced algal growth and increased water column transparency due to lower algal cell concentrations and, to a lesser extent, the addition of highly transparent water to the lake volume. Increased transparency, in turn, should lead to the proliferation of more desirable rooted aquatic plants (NYSDEC, 1990).

Algal concentrations may be reduced by flushing alone (e.g., the discharge of lake water). If the flushing rate is greater than the algae growth rate, algal cells may be washed out of the lake system. Control can be achieved by a flushing rate of approximately 10-15 percent per day (NYSDEC, 1990). If flushing alone can be used to decrease algae concentration through washout, then lower quality water can be used, provided that the increases in algal growth rate resulting from the higher nutrient concentrations are not sufficient to exceed the increased flushing rate. Unfortunately, given the lack of an unlimited external supply of dilution water in the Lake Tarpon watershed, flushing rates approaching 10-15 percent per day are not considered achievable. In addition, dilution water with nutrient concentrations higher than those in the lake may exacerbate the existing water quality problems.

Using mean annual TN and TP concentrations from 1995 water quality data from Lake Tarpon, it is estimated that the discharge of 1.0 foot of water (e.g., from elevation 3.0 to 2.0 NGVD) associated with an enhanced lake level fluctuation schedule would result in a nutrient mass discharge of 4.41 tons of TN and 0.25 tons of TP. Although lake retention time would be slightly reduced, most of the discharged nutrient mass would be replaced by nutrients contained in the inflowing precipitation, runoff and groundwater. Effective dilution of the in-lake nutrient mass would occur only if the cumulative nutrient concentrations in the inflow waters were even slightly lower than in-lake concentrations, but measurements of the nutrient concentrations of inflowing waters indicate that only precipitation is less concentrated than lake water with respect to TN and TP. For this reason, it is imperative that a source of high quality dilution water be used in Lake Tarpon.

Macrophyte Harvesting - Mechanical harvesting is not only effective at controlling nuisance aquatic vegetation, but it can also be used as a means to improve water quality problems related to eutrophication. The growth of aquatic macrophytes requires the assimilation of both water column and sediment nutrients. Physical removal (i.e., harvesting) of the plant biomass is highly effective in preventing the return of the assimilated nutrients to the water column or sediments as the plants decompose.

Interest in the use of aquatic plants for eutrophication management has increased sharply in the past few years, accompanied by an emphasis on the use of naturally occurring rooted macrophytes for removing both water column and sediment nutrients. There have been several reports published on the successful application of mechanical harvesting of rooted aquatic plants to the mitigation of eutrophication (Souza, et. al., 1988; Frederiksen, 1987).

Mechanical harvesting can directly reduce the coverage of both submergent and emergent nuisance aquatic vegetation. In addition, it will contribute to the removal of nutrients from the lake ecosystem. Using cattail tissue analysis data from Lake Tarpon (Dames & Moore, 1992), the harvesting of 10 acres per year of cattails would result in the removal of approximately 170 tons of dry weight organic matter, and 0.3 tons of TP, from the system.

Based on available harvesting data from Lake Okeechobee (Gremillion et al., 1988), it is estimated that the controlled harvest of approximately 100 acres of hydrilla in Lake Tarpon could result in the annual removal of approximately 12 tons of TN and 1.5 tons of TP per year. If this mass of plant tissue were to senesce and decompose simultaneously, as would be the case after a large scale chemical treatment, the harvesting of this material would result in a very substantial internal load reduction which is equal to approximately 20 and 9 percent of the total annual external TN and TP loads, respectively.

Summary of Expected Pollutant Load Reductions

Based on the analysis conducted by PBS&J (1998), external and internal load reductions for nitrogen and phosphorus were identified. These load reductions are summarized in Table A-6.