St. Johns River Water Supply Impact Study

byGetachew Belaineh Ph. D., P.H.1

Brian McGurk P.G.1

Louis Motz Ph. D., P.E2

Follow up Review meeting March, 2010

1 St. Johns River Water Management District2 University of Florida, (Expert assistance)

Investigation of the interaction between St Johns River and the underlying aquifers in

the Middle Basin

The National Academies Follow Up Remarks

1. Show the stability of chloride concentration more explicitly and for more locations.

2. What is the criteria used to determine whether the change due to density gradient is significant or not?

3. (a) The committee is looking forward to further analysis that might rule out the need for transient

groundwater model.

(b) How is the steady state assumption considered conservative?

Remark #1Factors used in evaluating the stability of chloride concentration in the aquifer

SJR Basin Location of

Hydrogeologic Cross-Section & Data Points

04/19/23 B. McGurk, GWP WSIS support

Ground Water Flow System: Orlando through USJRB(figure 25 from Tibbals, 1990 {USGS Water Supply Paper 1403-E})

04/19/23 B. McGurk, GWP WSIS support

Methods used to estimate chloride loading to SJR

• Data obtained from monitoring wells and chloride

concentration map produced by the District.

• GIS was used to create a raster and calculate chloride concentration for each ECF model grid.

• Chloride loading to SJR was estimated as a product of the flux calculated by ECF and chloride concentration calculated by the above method.

Verification of stability of chloride concentration in the aquifer

• Data from eight monitoring in the upper and middle basins used.

• Period of record mostly 1990 - 2009

Location of monitoring wells from which data was obtained for the analysis

Chloride concentration in one of the monitoring wells, S-0025

Chloride concentration in one of the monitoring wells, L-0032

Statistical analysis resultsData mostly covers 1990-2009

Well ID

Mean Concentration

(mg/L)Variability about

mean (%)

BR-1526 1722 1.9S-0025 5235 1.6V-0083 3198 1.7L-0032 753 0.7V-0818 663 0.8S-1397 42 1.5V-0165 28 1.3L-0455 9 2.2

Data showed noticeable spatial variability, but temporal variability is consistently insignificant at all the locations the test was conducted. Temporal variability about the mean ranges between

0.7 and 2.2%.

Laboratory uncertainty ± 10 – 15%. (Discussion with staffs of District Laboratory and Columbia Analytical Service, Inc. )

Tibbals (1990) documented chloride concentration in Floridan in the SJR area is temporally constant.

(U.S. Geological Survey Professional Paper 1403-E)

Summary on the stability of chloride concentration in the aquifer near SJR

Remark #2Factors used in evaluating the significance of the impacts resulting from potential density stratification effects

Calculated effect of density stratification using monitoring wells data

(calculation procedure presented in the previous meeting)

Well ID

Well Depth

(m)

Average Concen. (mg/L)

Potentiometric surface elevation (m_NGVD29) Flux –m3 s-1

Uncorrected Corrected Uncorrected Corrected

S-0025 47 5220 2.3 2.4 0.15

0.16

V-0083 132 4047 2.0 2.3 0.08 0.10

BR-1526 91 1689 6.4 6.5 0.02 0.02

Summary on salinity change due to density stratification

Salinity change due to density stratification in

Lake Harney area, which is one of the areas

with highest Cl concentration, is less than

0.012 psu (6.6 mg/L Cl).

Remark #3Factors used to evaluate the difference between steady-state and transient ground water discharge and loading to

the Middle St. Johns River

Since the primary purpose of the groundwater model is to provide boundary condition to the MSJR hydrodynamics and salinity model, sensitivity of the model is considered as the best indicator whether or not transient flux is needed.

Evaluation Approach

Transient interflow, Q(i,t), between SJR and Floridan was estimated using the following relationship:

Q(i,t) = conductance (Hriver (i,t) - Haquifer (i,t))

Where …. Hriver (i,t) = River stage near monitoring well i at time t. Haquifer (i,t) = Aquifer head in monitoring well i at time t. conductance = Aquifer property from ECF model.

Conceptual Model of Surface & Ground Water Interaction

Location of monitoring wells from which data was obtained for the calculation

Comparison of surface and groundwater discharges at the southern EFDC model boundary

Middle St. Johns River system was simulated with EFDC for the period of 1995 – 2005 to compare the effect of steady state and transient groundwater fluxes on water level and salinity using the following conditions:

1. Steady state groundwater flux (from ECF).I. With 155 MGD withdrawalII. Without withdrawal

2. Transient groundwater flux (estimated using data).I. With 155 MGD withdrawalII. Without withdrawal

Hydrodynamics and salinity simulations

The river response, water level and salinity, were evaluated at the following five locations in the river.

• SR46 Bridge south of Lake Harney• Lake Jesup• US17 Bridge north of Lake Monroe• SR44 Bridge near DeLand• SR40 Bridge near Astor

Model outputs

Model output locations

Difference between response of the river to steady state and transient fluxes without the proposed withdrawal

Difference between response of the river to steady state and transient fluxes

with the proposed 155 MGD withdrawal

Histogram showing water level difference between the steady state and transient fluxes

with the 155 MGD withdrawal

μ = 0.03038 cmσ. = 0.20159 cm

μ = 0.02775 cmσ. = 0.20742 cm

Histogram showing water level difference between the steady state and transient groundwater flows

without withdrawal

Histogram showing bottom layer salinity difference between the steady state and transient fluxes

with the 155 MGD withdrawal

μ = 0.00000 psuσ. = 0.01440 psu

μ = 0.00000 psuσ. = 0.01440 psu

Histogram showing top layer salinity difference between the steady state and transient fluxes

without withdrawal

Response of the river to the steady state & transient groundwater flow match.

• Water level: 95% of the time differences ≤ 0.489 cm• Salinity: 95 % of the time differences ≤ 0.035 psu

Steady state results capture the temporal variability adequately.

The correlation between the steady state and transient responses of the river exceeds 0.95; significant at 0.01 level.

Summary of the response of SJR to steady state and transient

groundwater flows

What makes steady state a conservative estimate?

• In three of the four wells the mean of the transient groundwater flow estimate was lower than the steady state estimate for the cells the wells are located. Well S-1397 is the exception.

Conclusions

1. Chloride stability: Data showed although there exists noticeable spatial variability, temporal variability is negligible.

2. Density stratification effect: Considering the highest Cl concentration data, the correction for density gradient did not show noticeable change in the river salinity.

Continued….conclusions

3. (a) Steady state/Transient Analysis: Statistical analysis of the results of EFDC model sensitivity test showed the assumption of steady state is valid for the purpose of the groundwater model.

(b) Estimation of groundwater flow under steady state assumption is not always higher than transient condition. In this study comparisons showed the steady state groundwater flow for the 1995 hydrologic condition higher than the average transient flow for the period of 1995 – 2005 at three out of four locations.

THANK YOU