Michael J. BraytonMichael J. Brayton

MD/DE/DC Water Science CenterMD/DE/DC Water Science Center

Hydrologic Controls on Nutrient and Pesticide Transport through a Small Agricultural Watershed, Morgan Creek, Maryland, USA

U.S. Department of the InteriorU.S. Geological Survey

Morgan Creek watershed is located in the northern part of the Delmarva Peninsula

ACT is a water-budget approach to sampling nutrients and pesticides

Define the conceptual hydrology and identify transport pathways

Sample stream water chemistry, which represents an integration of all source water inputs

• Part of the National Water Quality Assessment (NAWQA) trends network

• Part of the NAWQA topical team study of Agricultural Chemical Transport (ACT)

Morgan Creek Watershed

Physical Setting:• 31 square kilometer area (12.7 square miles)

• 90% agricultural

• Soils moderately well-drained, porous sandy loams

• Underlying aquifers are marine sand, silt, and clay

• Minimal irrigation with no inter-basin transfer

•Humid sub-tropical climate

•Elevation range is sea level to 25 meters

•Existing stream gage with long-term record

Morgan Creek Watershed- Annual Water Budget

GW= ground-water loss to adjacent basin

= 2 cm/yr

• Estimated from ground-water model

SW = Stream flow

= 32 cm/yr

• Estimated from long-term stream flow record

Budget Equation: PPT = ET + GW + SW

112cm = 78cm + 2cm + 32cm

ET = Evapotranspiration

= 78 cm/yr

• Calculated from budget equation

PPT = Precipitation

= 112 cm/yr

• 8-10 cm/month, evenly distributed

April May June July August

PRE-APPLICATION

TRIED TO SAMPLE EVERYEVENT, EARLY ON

1

2,3

4

5 6

8

7

9

MIS

SIN

G R

EC

OR

D

9 storms were sampled in 2003

MAJORAPPLICATION

Sampling Approach

Use a combined approach:

Fixed interval sampling at regular flows throughout the year (2002-04)

Targeted storm sampling during growing season(2003, 2004)

Analyzed for: pesticides, nutrients, sediment, TOC, DOC, major ions

How does the rainfall-runoff process work in the watershed?

How is stream chemistry affected by changes in season or stream flow?

Str

eam

Dis

char

ge

(cfs

) an

d

Sp

ecif

ic C

on

du

ctan

ce (

uS

/cm

)

Contributing Sources of Water

Stream water is Stream water is a variable mix a variable mix of different of different sourcessources

Sampled other Sampled other hydrologic hydrologic compartments compartments for chemical for chemical signaturessignatures

Stream

Deep Ground Water

Shallow Ground Water

Precipitation

At what flow does runoff become important?

Stream Discharge (CFS)Sus

pend

ed S

edim

ent

Con

cent

ratio

n (m

g/L)

Runoff begins around 12cfs

Storm Related FlowBase Flow

WINTER SPRING SUMMER AUTUMN

Base flow accounts for 60% of the annual flow at Morgan Creek

Less soluble compounds move in runoff

Concentrations can also vary by season

For Morgan Creek, these include: ammonia, organic nitrogen, phosphorus, suspended organic carbon, pesticide parent compounds

Stream Discharge (CFS)

Concentrations vary by stream discharge

Spring

Summer

Am

mon

ia +

Org

anic

Nitr

ogen

(m

g/L

as N

)

WINTER SPRING SUMMER AUTUMN

Runoff begins

More soluble compounds move in base flow

Nitrate is primarily from groundwater

For Morgan Creek, these include: nitrate and other major ions, iron, bicarbonate, pesticide degradates

Nitr

ate

+ N

itrite

, fil

tere

d (m

g/L

as N

)

Nitrate is present all year and at all flows

Stream Discharge (CFS)

WINTER SPRING SUMMER AUTUMN

Runoff begins

Dynamic chemistry during storm events

Potassium

Nitrate + Nitrite

Particulate Nitrogen

Con

cent

ratio

n (m

g/L)

May 25 May 26 May 27 May 28

3:00 15:00 3:00 15:00 3:00 15:00 3:00

Variable transport behavior during storm events

Metolachlor-ESA

Atrazine

Metolachlor-OA

Metolachlor

DEA

July 12-14, 2004 at Morgan Creek

Parent pesticides are transported primarily in runoff

Metabolites of metolachlor are transported in ground water

However

DEA, a metabolite of atrazine, is transported in runoff similar to its parent compound

4:00 16:00 4:00 16:00 4:00 16:00 4:00

C

once

ntra

tion

(ug/

L)

C

once

ntra

tion

(ug/

L)

Summary

Stream chemistry reflects flow-dependent contributions Stream chemistry reflects flow-dependent contributions from many source watersfrom many source waters

Insoluble compounds tend to travel in runoff, at stream Insoluble compounds tend to travel in runoff, at stream flows greater than 12 cfsflows greater than 12 cfs

Soluble compounds can travel in both runoff and base Soluble compounds can travel in both runoff and base flow (primarily ground-water discharge)flow (primarily ground-water discharge)

Pesticide and nutrient loads are greatest in storms after Pesticide and nutrient loads are greatest in storms after application, usually in May and Juneapplication, usually in May and June

Pesticides and their metabolites are present year round Pesticides and their metabolites are present year round in streams and can show contrasting transport behaviorin streams and can show contrasting transport behavior

Watershed Management Implications

Develop a conceptual hydrologic understanding to focus Develop a conceptual hydrologic understanding to focus monitoring strategies monitoring strategies

Build a robust data set by sampling all seasons at Build a robust data set by sampling all seasons at normal flow and during storm eventsnormal flow and during storm events

Use an iterative process of monitoring to identify Use an iterative process of monitoring to identify hydrologic transport pathways hydrologic transport pathways

Target specific management strategies based on primary Target specific management strategies based on primary transport pathways transport pathways

Remember: Remember: Hydrologic residence times may strongly Hydrologic residence times may strongly affect when you begin to see changes! affect when you begin to see changes!

THE ENDTHE END

Contact: Mike Brayton (Contact: Mike Brayton (mbrayton@usgs.govmbrayton@usgs.gov), 410-238-4315), 410-238-4315