TMDL Definitions

TMDL Data Center Definitions

Definitions of terms and abbreviations used in the Maryland TMDL Data Center

Aggregate Allocation – An aggregate allocation is the summation of pollutant loads from multiple NPDES-permitted sources.  These allocations and their associated reductions are not expected to be met by an individual facility, but rather must be achieved by the combined activities of every facility contained in the aggregate. Please contact MDE if you have any questions regarding an aggregate allocation assigned to a facility.

BOD – Biochemical oxygen demand

CaC03 – Calcium carbonate

CBOD – Carbonaceous biochemical oxygen demand

Chesapeake Bay Segment (or segmentshed) – The watershed scale at which the Chesapeake Bay TMDL and WIPs were modeled. There are 58 Chesapeake Bay segments in Maryland.

Delivered Load – The amount of a pollutant load transported annually from a source to the tidal waters of the Chesapeake Bay. These loads are generally lower than EOS loads and reflect losses during transport from the stream edge to the Bay.

EOS Load (Edge-of-Stream Load) – The amount of a pollutant load transported annually from a source to the nearest stream.

GIS ID – A reference number that relates database data to map data. For example, according to a statewide TMDL search for nitrogen, the local TMDL for the Baltimore Harbor, has a GIS No of G2390, which corresponds to a polygon in the map of approved local nitrogen TMDLs.

Individual Allocation – An individual allocation is a pollution load limit that is assigned to a single NPDES-permitted entity.

LA (Load Allocation) - A load allocation is the maximum load of a pollutant emanating from non-permitted or upstream sources that can enter a particular waterway without violating applicable water quality standards.

MS4 – Municipal Separate Storm Sewer System

NBOD – Nitrogenous biochemical oxygen demand

PCBs – Polychlorinated biphenyls

Planning Target (Aggregate or Individual) – The 2025 target load for a NPDES permittee set out in the Phase II WIP Final Targets.

Phase II WIP Maryland’s Phase II Watershed Implementation Plan for the Chesapeake Bay Watershed

SHA – Maryland State Highway Administration

TN – Total nitrogen

TP – Total phosphorus

TSS – Total suspended solids

UT – Unnamed tributary. For example, "UT Laurel Run" is an unnamed tributary of Laurel Run.

WLA (Waste Load Allocation) - A waste load allocation is the maximum load of a pollutant emanating from regulated point source sectors that can enter a particular waterway without violating applicable water quality standards.

 

For information about the TMDL Data Center, please contact Greg Busch at 410-537-3901 or at gregory.busch@maryland.gov.