Conductivities of fractured limestone aquifers (m/s)

0.40

0.35

0.30

0.25

0.20

0.15

0.10

0.055 6 7 8 9

0.12 3 4 5 6 7 8 9

12 3 4 5 6 7 8 9

10

Conductivities of fractured limestone aquifers (m/s)

Tor

tuos

ity (

-)

Walkerton, Ontario (CANADA)

Bari (ITALY)gasification site

Bari - IRSA West – Central Florida (USA)Barton Springs,

Texas (USA)

0 2 0 4 0 6 0 8 0 1 0 0 1 2 0 1 4 0

2 0

4 0

6 0

8 0

1 0 0

1 2 0

1 4 0

1 6 0

p z 2

p z 6

p z 4

p z 5

p z 8

p z 9

p z 1 0p z 1 1

p z 1 2

PT1

Tortuosity: 0.24

MT3DMS

MT3DMS modified

IHS

Dispersivity: 10m

pz1

s19

Total phenols (26.7 mg/L) during 2004 in the sampled groundwater

from well PT1

Tot

al p

heno

ls (

mg/

L)

Time (d)

0 20 40 60 80 100 120 140 1600

20

40

60

80

100

120

140

160

pz1 pz2

pz6

pz4

pz5

pz8

pz3

pz7

pz9

pz10pz11

pz12

PT1

INTRODUCTIONINTRODUCTIONModel predictions of flow and pollutant transport in fractured rocks are subject to uncertainties due to imprecise knowledge of the position, orientation, length, aperture and density of the fractures. These properties are difficult to quantify precisely because fractures are located in depth in subsoil and, generally, tectonic and stratigraphic studies may provide only fracture frequency and their orientation. The use of the “equivalent” continuum models might help hydro-geologists to solve flow and pollutant transport problems in fractured aquifers, when fracture properties are unknown. Test results have shown a delay of velocity estimated using continuum models, with respect to the discrete model, that decreases by increasing the hydraulic conductivity of the limestone aquifer under consideration. Maximum discrepancies have been noted for conductivity (< 10-4 m/s) typically associated with non - karst limestone aquifers. The tortuosity has been then included into the codes in order to address flow velocity calculations in numerical codes, such as MT3DMS (Zheng, C. 2010).

TORTUOSITY/CONDUCTIVITY RELATIONSHIPTORTUOSITY/CONDUCTIVITY RELATIONSHIP

RE

FE

RE

NC

ES

RE

FE

RE

NC

ES

STUDY AREASTUDY AREA

x32U

2c

x12

bU

2

The known analytical solutions of the NS equations can be defined in very simplified conceptual models (Figure 2), under laminar flow. The presented (Figure 2) conceptual models assume that matrix blocks are impermeable and, similarly to MT3DMS, contain a dual domain mathematical representation to approximate the effects of a fast system (fractures) and an immobile system (rock matrix).

An improvement of the fissure model has been achieved. By including a tortuosity factor [-] to define b and δch [L], i.e. the apparent enlarged fissure aperture or tube size, as

n

k12ch

k: permeability n: porosity

The tortuosity is a fundamental parameter in describing the complexity of the path-line of water flow propagating within a single fracture or a porous medium. That tortuous paths play an important role in affecting flow in a rough fractures was experienced by Tsang (1984). An improved solution of MT3DMS was obtained by introducing the tortuosity.

n

k1b

2

NAVIER–STOKES SOLUTIONSNAVIER–STOKES SOLUTIONS

and by USING TORTUOSITY…and by USING TORTUOSITY…

These equations provide different water velocities when a set of parallel fractures is modified in an "equivalent" tube model, by reducing size of the tube diameter and increasing the tube number. When the conceptual model of groundwater is not properly selected velocity underestimation will occur.

CAN EQUIVALENT CONTINUUM MODELS SIMULATE FLOW AND POLLUTANT TRANSPORT IN

FRACTURED LIMESTONE AQUIFER?EGU 2011 HS8.2 - 1407

C. Masciopinto (1) (costantino.masciopinto@ba.irsa.cnr.it) and D. Palmiotta (1,2) (domenico.palmiotta@ba.irsa.cnr.it)

(1) Water Research Institute of National Research Council, IRSA-CNR , Bari, Italy – (2) Polytechnic University of Bari, Italy

CONCLUSIONSCONCLUSIONS

Figure 1. Study area

Figure 2. Conceptual modelCase test of the Bari contaminated siteCase test of the Bari contaminated site (Southern Italy)(Southern Italy)

Figure 3. Stratigraphic section of the study area

Figure 5. Tracer breakthrough curve and expected concentrations given by IHS (Masciopinto et al., 2008), MT3DMS and MT3DMS modified codes

92 m/d

Water velocityscale:

3.5 m/d

Contour heads (m)during winter 2002

Apparent pollutant pathways

Pollution sourcesObservation Wells

Monitored Wells

Contour heads (m) during winter 2002 in an equivalent porous media

Pumping tracer testPumping tracer test

Schematic cross-section in the fractured limestone of the tested area

BariAPULIA

20 m

Injection well #E

well #C

Figure 4. Study area of the field test in Bari (IRSA-CNR)

Figure 6. Apparent contaminant pathways (outputs) using IHS and the particle tracking code.

Calcarenite di Gravinap.c.

- 4 m

-34 m

-54 m

(Cretaceous)

Calcare di Bari

Jurassic fractureddolomite

Figure 7. Velocity vectors derived from MODFLOW

Figure 8. Best-fit of values obtained by comparing the results of tracer tests in some fractured limestone aquifers

Tracer: chlorophyll V: 200 L of solution C: 0.5 g/L

72.5 L/s

s19

pz1

PT1

pz3

pz7

1.0

0.8

0.6

0.4

0.2

0.0

12 3 4 5 6 7 8 9

102 3 4 5 6 7 8

C/Cmax

Time (min)

Figure. 8 Expected (model) outputs and observed concentrations during winter 2004 in well PT1 at the contaminated site with conductivity 0.01 cm/s, porosity 0.003 and tortuosity 0.24.

IHS output

Modified MT3DMS

Chlorophyll

MT3DMS

As the best conceptual model must reproduce the fissure geometry (i.e. apertures, number, orientation, etc.) of the real medium as closely as possible, tortuosity must be included in conductive tube models in order to explain groundwater velocities. By using tortuosity in conductive tubes, underestimations of groundwater velocities in an equivalent continuum model (i.e. porous medium) can be eliminated. Successful simulations of flow and pollutant transport have been carried out at the Bari fractured aquifers by using tortuosity.

Alluvial deposits (sand)

Lame

Calcarenite di Gravina (Lower Pliocene)

Calcare di Bari (Cretaceous)

roads railroadZheng, C. 2010. MT3DMS v 5.3: Modular three-dimensional multispecies transport model for simulation of advection, dispersion and chemical reactions of contaminants in groundwater systems Supplemental User’s Guide. Department of Geological Sciences, The University of Alabama Tuscaloosa, Alabama 35487, Technical Report, February 2010.

Tsang, Y. W. 1984. The effect of tortuosity on fluid flow through a single fracture. Water resources research, 20 (9), 1209-1215.

Masciopinto, C., La Mantia, R. and C.V. Chrysikopoulos 2008. Fate and transport of pathogens in a fractured aquifer in the Salento area, Italy. Water Resources Research, 44, W01404,doi:10.1029/2006WR005643.

p.c.

16.2 m above sea

2.2 m above sea

0.3 m

Water velocityscale:

35 m/d

0.3 m/d