The Port Tobacco River TMDL

View of Port Tobacco River
View of Port Tobacco River

In  1996, the Port Tobacco River was identified as being impaired due to signs of eutrophication, the overenrichment of aquatic systems by excessive inputs of nitrogen and phosphorus. Eutrophication was evidenced by recurrent seasonal algal blooms. Highly eutrophic waters will characteristically have fewer species present, and dissolved oxygen content will fluctuate between day and night, which can cause fish kills. Total Maximum Daily Loads (TMDLs) for the Port Tobacco River for the nutrients nitrogen and phosphorus were established in 1999.

The objective of the TMDL for the Port Tobacco River is to reduce nitrogen and phosphorus inputs to a level that will ensure the maintenance of dissolved oxygen standards and reduce frequency and magnitude of algal blooms, allowing the designated uses of the Port Tobacco River to be met. Specifically, the TMDL is intended to assure that a minimum dissolved oxygen level of 5 mg/l is maintained throughout the Port Tobacco system, and to reduce peak chlorophyll a levels (a surrogate for algae blooms) to below 52 μg/l.

The TMDL includes Wasteload Allocations (WLAs) to describe pollutant loads from point sources (individual man-made sources of direct discharge to water, such as pipes and ditches), and Load Allocations (LAs) to describe pollutant loads from other, nonpoint sources, such as runoff from developed areas.

The TMDL also takes into account critical conditions for the Port Tobacco River. Low stream flow conditions that typically occur in August, when nutrient concentrations are higher and water temperatures are warmer, create favorable environmental conditions for algal growth. Persistent seasonal algal blooms have been documents in the reach near the Route 6 crossing. Mean summer concentrations of chlorophyll a in that region typically fall in the 45-65 μg/l range, with nuisance algal bloom levels periodically reaching 150 to 200 μg/l. The low-flow TMDL is designed to maintain water quality standards during low-flow conditions.

The TMDL recognizes a second critical condition when increased precipitation events increase nonpoint source loads of nutrients, which could adversely affect water quality. The annual TMDL based on average flow conditions takes these increased loads into account.

Summary of TMDLs

Parameter TMDL LA WLA MOS FA Type/Period
Nitrogen (lb/month) 8,710 5,776 1,597 173 and implicit 1,164 Low-flow
May 1 – October 31
Phosphorus (lb/month) 871 696 88 21 and implicit 66
Nitrogen (lb/year) 243,310 190,470 24,920 5,840 and implicit 22,080 Average-flow Annual
Phosphorus (lb/year) 15,570 12,500 1,060 400 and implicit 1,610
  • LA=Load allocation (nonpoint source)
  • WLA=Waste load allocation (point source)
  • MOS=Margin of safety
  • FA=Future allocation

Wasteload Allocations (WLAs)

Individual WLAs are established through NPDES permits which will be issued, reissued, or modified as appropriate on a watershed-wide basis.

Summary of low-flow and average-flow TMDL WLAs

Low-flow TMDL WLAs Average-flow TMDL WLAs
Source Permit # TN Load (lb/month) TP Load (lb/month) TN Load (lb/month) TP Load (lb/month)
La Plata MD0020524 1,355 68 21,970 820
Charles Community College MD0052311 124 7 1,510 80
Mt. Carmel MD0053228 77 9 930 100
Thunderbird Apts. MD0050334 42 5 510 60
Existing WLA 1,597 88 24,920 1,060
Maximum Allowable Point Source Load 2,761 154 42,720 1,870
Future Allocation 1,164 66 17,800 810

The individual WLA for La Plata assumes that Biological Nutrient Removal (BNR), which will be implemented through the NPDES permit, will achieve a total nitrogen (TN) concentration of 6 mg/l at maximum discharge flow during low-flow conditions and 8 mg/l, the anticipated average annual concentration, during average-flow conditions. Chemical Phosphorus Removal (CPR), already implemented at La Plata, accounts for the relatively small contribution of phosphorus from this facility. The other 3 point source discharges were considered too small to impose nutrient limitations. The loading from these facilities assumed the maximum allowable nitrogen and phosphorus discharge concentrations without BNR and CPR at maximum flow for both flow conditions.

Load Allocations (LAs)

The TMDL does not assign specific load allocations to each particular land use. Instead it includes a gross load allocation for the low-flow and average-flow TMDLs.

The LAs for the low-flow TMDL represent actual instream values determined from two instream data stations in Port Tobacco River during August 1984. Under low-flow conditions, nonpoint source loads are mainly due to groundwater and not attributable to any particular land use.

Nonpoint sources for the average-flow TMDL were calculated using a simple land use area/loading coefficient approach based on three land use types (urban, agriculture, and forest) and the year 2000 loading rates from the Chesapeake Bay Model which accounts for atmospheric deposition and septic tanks. Background contributions as calculated from the low-flow TMDL are included.

Low-flow TMDL LAs Average-flow TMDL LAs
Nitrogen (lb/month) Phosphorus (lb/month) Nitrogen (lb/month) Phosphorus (lb/month)
Load Allocation 5,776 696 190,470 12,500

Assurance of Implementation

The TMDL document states that, for the low flow TMDL, which is driven primarily by point source loads, NPDES permits will play a major role in assuring implementation. For the average annual TMDLs, which involve more significant nonpoint source considerations, Maryland would rely on the Tributary Strategy for Nutrient Reduction developed as a result of the 1987 Chesapeake Bay Agreement and the Nonpoint Source Management Plan required by section 319 of the Clean Water Act, the traditional NPDES program, and periodic state evaluation of the TMDL.