Testing the Waters: Relationships Among Land Use, BMPs, and Water Quality

Ryan Corcoran (Government)

The full report for this study is available here. Selected excerps are available below.

INTRODUCTION
The College Creek watershed is relatively small—13.7 square miles—but includes a broad range
of land uses (e.g., housing subdivisions, golf courses, shopping centers, a college, an airport,
agricultural lands) that are potential sources of non‐point discharges of sediment and nutrients.
Associated with these different land use types are management practices designed to reduce
the runoff of pollutants (e.g., wet retention ponds, dry ponds, filtration swales, rain gardens).
Indeed, the federally mandated TMDL “diet” for tributaries to Chesapeake Bay requires local
accounting for some regions showing how land‐based management practices will reduce runoff
of suspended sediment, nitrogen, and phosphorus. Unfortunately, the “downstream” impact
of these watershed implementation plans—for practical and logistical reasons—has not always
been determined in the field, making it difficult to assess whether pollution reduction targets
will be met.

We proposed to examine the spatial arrangement of land use and best management practices
in the College Creek watershed and determine the relationship of these land‐based variables to
long‐term measures of water quality. Our primary objective was to compare a more complex,
data‐based model of sediment and nutrient export from sections of the College Creek
watershed with a simpler model of export based on edge‐of‐stream loading rates. With this
field test of the simple model we hope provide guidance to municipalities regarding the
potential impact of local TMDL action plans.

METHODS
We used data from the ongoing water quality monitoring program for the College Creek
Alliance, initiated as part of a VEE‐sponsored program beginning Fall 2004. For that program,
nine streams, eight ponds and six tidal creeks in the College Creek watershed are monitored
quarterly for water quality at base flow (during non‐storm flows).

The overall goal of this project was field verification: to determine whether the simple estimate
of annual sediment and nutrient discharge using TMDL loading rates agreed with a more
complex SWMM estimate garnered from field measures of precipitation and water quality.
None of the 17 subwatersheds exhibited agreement with respect to nitrogen, and only two
subwatersheds (17 and 20) showed reasonable agreement for both TSS and total P. Even with
the general disagreement between methods, it is important to consider these differences
among subwatersheds because reductions in nutrient and sediment discharge from the College
Creek watershed are part of the three‐phase, Chesapeake Bay TMDL watershed
implementation plan. Much of the watershed includes Municipal Separate Storm Sewer
Systems (MS4) where stormwater retrofits and other implementation plans will be required to
meet required reductions in nutrient and sediment discharge. Reduction targets from MS4
permit areas will be 5% by 2018, 40% by 2023, and 100% by 2028. Our data provides the
antecedent conditions prior to watershed implementation plans in these regulated MS4
permitting areas in the College Creek watershed, against which future load reductions can be
compared.

Selected Discussion: