Fall Line Input Monitoring on the Potomac River at Chain Bridge

Transcription

1 Fall Line Input Monitoring on the Potomac River at Chain Bridge Metropolitan Washington Council of Governments and Occoquan Watershed Monitoring Laboratory Regional Monitoring Subcommittee Meeting 15 August 2007

2 History of the Station - I Originally operated by USGS Difficulty in maintaining automatic sampler operations Manual sampling (USGS cross-section integrated method) very difficult to use during high flow events MWCOG/OWML assumed operation in 1983 Storm sampling conducted under assumption of a well-mixed flow regime at Chain Bridge due to narrowing of Potomac River at that point Storm sampling originally was by collection of discrete samples triggered remotely (via phone from OWML) Flows obtained via telephone access to Little Falls gage Occoquan Watershed Monitoring Laboratory 2

3 History of the Station - II In January 1986, OWML converted to computer-controlled automated station Storm samples were flow-composited in situ using OWML-developed system Monitoring of Stream Hydrology and Quality using Microcomputers, Proceedings, Symposium on Monitoring, Modeling, and Mediating Water Quality, AWRA, Apart from equipment upgrades, this is basically the same system used today Occoquan Watershed Monitoring Laboratory 3

4 Objectives Locate, instrument, and operate a stream gaging and water quality sampling station for the Potomac River at the fall line Measurement of constituent loads delivered to the Potomac Estuary Enable comparison of point and nonpoint source loads in watershed Two candidate sites Potomac River at Little Falls Dam Potomac River at Chain Bridge Occoquan Watershed Monitoring Laboratory 4

5 Little Falls Long-term gaging record available March Present Good hydraulic control (Little Falls Dam) Rating table maintained by USGS Poor sampling location River is wide Point sampling to characterize loads not practical Flow patterns and mid-stream islands require cross-section sampling approach Access and safety issues Occoquan Watershed Monitoring Laboratory 5

6 Chain Bridge Unsuitable site for stream gaging Variable backwater effects from tidal fresh portion of estuary Complications in developing low and moderate flow rating table Excellent site for sampling Narrow, well-mixed cross section Suitable for using point samples to characterize river flow Method validated against cross-section integration approach Occoquan Watershed Monitoring Laboratory 6

7 The Solution Use Little Falls for flow measurements and Chain Bridge for sampling Occoquan Watershed Monitoring Laboratory 7

8 Telephone lines Microcomputer datalogger Microcomputer datalogger Automatic sampler Pump control interface Constant head tank Potomac River at Little Falls Dam Pressure transducer for stage measurement Submersible pump Waste Line Little Falls/Chain Bridge Gaging/Sampling System at the Potomac River Fall Line Potomac River at Chain Bridge

9 Little Falls Dam Looking South Occoquan Watershed Monitoring from WAD Laboratory Pump Station 9

10 OWML datalogger Occoquan Little Watershed Falls Pumping Monitoring Station Laboratory 10

11 Phone Jack OWML Datalogger Occoquan Watershed Monitoring Laboratory 11

12 Pressure Transducer Installation at Little Falls Pump Station Pressure Transducer

13 Potomac River Looking Upstream from Chain Bridge Occoquan Watershed Monitoring Laboratory 13

14 Occoquan Watershed Monitoring Laboratory 14 Potomac River looking north from Virginia shore at Chain Bridge

15 Storm flow in Potomac Occoquan Watershed Monitoring Laboratory River at Chain Bridge 15

16 Pump Control Constant Head Tank Datalogger/ controller Composite Sampler Discrete Sampler Occoquan Watershed Monitoring Laboratory 16

17 YSI Multiparameter Probe

18 Composite Occoquan Watershed and Discrete Monitoring Samplers Laboratory 18

19 Pump discharge and power supply housing Station Drain Occoquan Watershed Monitoring Laboratory 19

20 Submersible pump and flexible intake line Occoquan Watershed Monitoring Laboratory 20

21 Current Sampling Program - I Manual collection of baseflow samples Grab samples Frequency Weekly: April to November Bi-weekly: December to March Occoquan Watershed Monitoring Laboratory 21

22 Current Sampling Program - II Automated collection of stormflow samples Attempt is to collect every storm Two side-by-side samplers One sampler performs compositing If equipment malfunctions, storm samples are collected and composited manually Discrete storm samples (5 per storm) are also collected for approximately 5 events per year via the second sampler (started in November 1995) Occoquan Watershed Monitoring Laboratory 22

23 Schematic of Sequential-Discrete Sampling Method Discharge, L 3 t -1 Manual construction of composite in laboratory after collection V 3 V 4 V 5 V 2 V 6 V 7 V 8 V 9 V 1 Time Occoquan Watershed Monitoring Laboratory 23

24 Schematic of OWML Storm Compositing Method (flow weighted) Automated construction of composite at station V 1 = V 2 = V n V 1 V n-1 V n Time Occoquan Watershed Monitoring Laboratory 24

25 Schematic of Discrete Storm Sampling Discrete samples retrieved on rising and falling limbs of hydrograph Occoquan Watershed Monitoring Laboratory 25

26 Current List of Parameters Measured Flow Dissolved oxygen Total hardness Total organic carbon Total suspended solids Total nitrogen Total soluble phosphorus Temperature ph Turbidity Dissolved organic carbon Nitrate and nitrite nitrogen Total soluble nitrogen Soluble reactive phosphorus Conductivity Total alkalinity Fecal coliforms and E. coli Chemical oxygen demand Ammonia nitrogen Total phosphorus Soluble reactive silica Occoquan Watershed Monitoring Laboratory 26

27 OWML Method of Computing Loads - I Between baseflow samples, concentrations are linearly interpolated (except for storm periods, which are handled separately). These concentrations are then multiplied by daily flows to get daily loads. For partial days (i.e., when a storm either commences or ends), only the non-storm flow is used for baseflow loads for that day. If an entire day consists of stormflow, then the baseflow load is zero (no baseflow separation used). Occoquan Watershed Monitoring Laboratory 27

28 OWML Method of Computing Loads - II Storm loads are computed by multiplying entire storm volume by composite flow-weighted concentration. Baseflow and stormflow loads are then totaled for each day, and can then be totaled by month or year or other time interval. Concentrations for missed storms are estimated by using a distribution of concentration versus flow for all measured storms. Occoquan Watershed Monitoring Laboratory 28

29 Sample Counts Average annual number of samples collected by OWML: Storm composites, : ~12/year (299 total) Ambient samples, : ~40/year (965 total) Discrete storm samples, : ~39/year (392 total) Total samples in 24 years ( ): 1,656 Occoquan Watershed Monitoring Laboratory 29

30 USGS Estimation Process Collection of grab samples per year, distributed randomly between base- and stormflow ESTIMATOR software used to compute estimated loads Occoquan Watershed Monitoring Laboratory 30

31 Selected Results Occoquan Watershed Monitoring Laboratory 31

32 Average Annual Flow Average Annual Flow (computed from daily flows) 30,000 Average Annual Flow, cfs 25,000 20,000 15,000 10,000 5, Occoquan Watershed Monitoring Laboratory 32

33 Seasonal Average Non-Storm Flow Average annual (base and storm) flow: 12,517 cfs Flow, cfs Occoquan Watershed Monitoring Laboratory 33

34 Seasonal Non-Storm Average: Turbidity Turbidity, NTU Occoquan Watershed Monitoring Laboratory 34

35 Seasonal Non-Storm Average: TSS TSS, mg/l Occoquan Watershed Monitoring Laboratory 35

36 Seasonal Non-Storm Average: Total Nitrogen TN, mg/l Occoquan Watershed Monitoring Laboratory 36

37 Seasonal Non-Storm Average: Oxidized Nitrogen Oxidized N, mg/l Occoquan Watershed Monitoring Laboratory 37

38 Seasonal Non-Storm Average: Total Phosphorus TP, mg/l Occoquan Watershed Monitoring Laboratory 38

39 Seasonal Non-Storm Average: Orthophosphate Phosphorus OP, mg/l Occoquan Watershed Monitoring Laboratory 39

40 Seasonal Non-Storm Average: Total Organic Carbon TOC, mg/l Occoquan Watershed Monitoring Laboratory 40

41 Average Annual Flow Average Annual Flow (computed from daily flows) 30,000 Average Annual Flow, cfs 25,000 20,000 15,000 10,000 5, Occoquan Watershed Monitoring Laboratory 41

42 Annual Suspended Sediment Loads Annual Suspended Sediment Loads at the Potomac Fall Line 1.0E E E+09 Sediment Load, lb./year 7.0E E E E E E E E Occoquan Watershed Monitoring Laboratory 42

43 Annual Total Nitrogen Loads Annual Total Nitrogen Loads at the Potomac Fall Line 2.0E E E+08 TN Load, lb./year 1.4E E E E E E E E Occoquan Watershed Monitoring Laboratory 43

44 Annual Total Phosphorus Loads Annual Total Phosphorus Loads at the Potomac Fall Line 2.0E E E+07 TP Load, lb./year 1.4E E E E E E E E Occoquan Watershed Monitoring Laboratory 44

45 Annual Flow Weighted Concentration Trends Annual Flow Weighted Concentration, mg/l Total N TP TSS Occoquan Watershed Monitoring Laboratory 45

46 Benefits of Current Program - I Provides real loading data with high sampling frequency, thus reducing uncertainty in estimates. The total loads in each storm can be computed with great accuracy. For those storms where discrete samples have been collected: Provides a way of determining if the flow hydrograph and load curves are in phase or not. Allows for determination of rising and falling limb loads. Occoquan Watershed Monitoring Laboratory 46

47 Benefits of Current Program - II Provides greater insurance against uncertainty, whether real or perceived. The sampling of all storms results in the outliers or non-normal storms being better described. Similarly, the sampling of non-normal years will be better described. Program stakeholders are in an improved position to assess trends in key pollutant loads. Occoquan Watershed Monitoring Laboratory 47

48 Current Status (2007) The Potomac monitoring program at Chain Bridge has collected a large database that can be used in various ways, and the monitoring program continues as before. OWML is in the process of making some field data available on the web. Plans to transition to GOES (geostationary operational environmental satellite) in near future. Occoquan Watershed Monitoring Laboratory 48

49 Parameters That May be Added in the Future The following are either required for Chesapeake Bay Water Quality Model (WQ) or Watershed (WS) Model Particulate inorganic phosphorus (WQ + WS) Particulate organic carbon (WQ) Particulate inorganic carbon (WQ + WS) Volatile suspended solids (WQ + WS) BOD 5 (WS) Chlorophyll a (WQ + WS) Particle size (WS + WS, once storm per quarter) In addition, particulate nitrogen is a parameter that AMQAW recommends for measurement. Occoquan Watershed Monitoring Laboratory 49