# Presentation: 247507

## Michael J. Brayton
MD/DE/DC Water Science Center

- MD/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 Interior**
**U.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)

## Physical Setting:

- Morgan Creek Watershed

-  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

**Notes:**
No plumbing in the basin!
Funding eliminated for gage with > 50 year 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**

**Notes:**
Construct a water budget and simple conceptual model; ET calculated from equation above

## Sampling Approach

**April           May           June          July        August**

- PRE-APPLICATION

- TRIED TO SAMPLE EVERY
- EVENT, EARLY ON

**1**

**2,3**

**4**

**5**

**6**

**8**

**7**

**9**

- MISSING RECORD

**9 storms were sampled in 2003**

- MAJOR
- APPLICATION

- 	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?

**Stream Discharge (cfs)**** and ****Specific Conductance (uS/cm)**

**Notes:**
Strong seasonal component to storm sampling
Both Q and SC are on the same axis

## Contributing Sources of Water

- Stream water is a variable mix of different sources
- Sampled other hydrologic compartments for chemical signatures

**Notes:**
Water samples were collected from all hydrologic compartments

## At what flow does runoff become important?

- Stream Discharge (CFS)

- Suspended Sediment Concentration (mg/L)

- Runoff begins around 12cfs

- Storm Related Flow

- Base Flow

** ****WINTER**  **SPRING  SUMMER  AUTUMN**

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

**Notes:**
Antecedent soil moisture is important.
What is the rainfall duration and intensity?

## 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**

- Ammonia + Organic Nitrogen (mg/L as N)

**WINTER** **SPRING  SUMMER  AUTUMN**

- Runoff begins

**Notes:**
Both ammonia and organic N move overland; Less applied in the summer.

## 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

- Nitrate + Nitrite, filtered (mg/L as N)

- Nitrate is present all year and at all flows

- Stream Discharge (CFS)

**WINTER** **SPRING  SUMMER  AUTUMN**

- Runoff begins

**Notes:**
Primary N sources are agricultural chemicals and manure

## Dynamic chemistry during storm events

- Potassium

- Nitrate + Nitrite

- Particulate Nitrogen

- Concentration (mg/L)

- May 25

- May 26

- May 27

- May 28

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

**Notes:**
Sustained input versus quick delivery

## Variable transport behavior during storm events

- 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**

-     Concentration (ug/L)

-       Concentration (ug/L)

**Notes:**
Atrazine and DEA show similar pattern

## Summary

- Stream chemistry reflects flow-dependent contributions from many source waters
- Insoluble compounds tend to travel in runoff, at stream flows greater than 12 cfs
- Soluble compounds can travel in both runoff and base flow (primarily ground-water discharge)
- Pesticide and nutrient loads are greatest in storms after application, usually in May and June
- Pesticides and their metabolites are present year round in streams and can show contrasting transport behavior

**Notes:**
Load Estimator models have been run on the watershed; data gaps for storms in “winter” season

## Watershed Management Implications

- Develop a conceptual hydrologic understanding to focus monitoring strategies
- Build a robust data set by sampling all seasons at normal flow and during storm events
- Use an iterative process of monitoring to identify hydrologic transport pathways
- Target specific management strategies based on primary transport pathways
*Remember: *Hydrologic residence times may strongly affect when you begin to see changes!
- THE END
- Contact: Mike Brayton (_[mbrayton@usgs.gov](mailto:mbrayton@usgs.gov)__), 410-238-4315_

**Notes:**
Example BMP: If shallow flow paths with high nitrate are discharging to riparian zone, then increase its size for enhanced denitrification.
Conversely, if long GW flow paths are discharging to SW, then it may take years to see changes in application practices.