USING 222Rn TO IDENTIFY AND QUANTIFY

GROUNDWATER INFLOWS TO THE MUNDO RIVER

(SE SPAIN)

INTERNATIONAL SYMPOSIUM ON

Revisiting Foundations and Exploring Frontiers Isotope Hydrology - CN225

L. Ortega a, b, M. Manzano b, E. Custodio c, J. Hornero d, J. Rodríguez-Arévalo e

a CONICET-IHLLA, Azul, Argentina. lortega@faa.unicen.edu.ar b Universidad Politécnica de Cartagena (UPCT), Cartagena, Spain. c Universidad Politécnica de Cataluña (UPC), Barcelona, Spain. d Instituto Geológico y Minero de España (IGME), Murcia, Spain. e Centro de Estudios y Experimentación de Obras Públicas (CEDEX), Madrid, Spain.

Vienna, Austria 11 – 15 May 2015

I. LOCATION & MOTIVATION

Upper Segura River Basin, SE Spain. Water stressed region that receives imported

water to supply high water demand areas. Identify and quantify groundwater discharge in

the basin is essential for the Water Action Plan of the Segura River basin Authority.

222Rn can be an useful tool to identify and quantify groundwater discharge in karstic aquifers.

Flow direction

II. OBJECTIVES

General aim:

To identify and quantify groundwater discharge to the Mundo River using 222Rn

(radon) activity.

Three main steps and objectives were established:

O1. To identify all the groundwater discharge areas existing along the studied

river stretch.

O2. To quantify groundwater discharge flows using radon mass balances (RMB).

O3. To check the performance of radon as a tracer to quantify groundwater

discharge to rivers in karstic areas by comparing Rn, Cl and total river flow

mass balances.

III. STUDY SITE. HIDROGEOLOGY

The area is folded and faulted, and tectonics is assumed to facilitate diffuse and concentrated groundwater discharge to the river.

Mainly fed by groundwater discharge from Mesozoic carbonate rocks, and

from several tributaries (only the Bogarra Creek is permanent).

3 surveys were conducted: October 2011, May 2012 and July 2013.

All surveys were planned at base flow conditions, though not fully achieved in 2013.

Chemical analysis, 222Rn measurements (≈ 1 h) and river flow gauging were performed at discrete locations and whenever it was possible.

• 17 river sampling/measurement. • 10 spring sampling/measurement.

222Rn was measured with a RAD7 and RADAQUA (Durrigde).

River flow was measured using an impeller flowmeter.

IV. MATERIALS

Groundwater discharge estimations by Rn mass balances (1) were performed for

discrete river volumes. Considering steady state conditions and assuming constant

concentration at each control section, good mixing inside the river volumen and no

rainfall during and in the days before the survey:

V. METHODS

𝑞𝐺𝑤 =𝑄𝑅𝑥𝑤

𝑑𝐶𝑅𝑑𝑥+ 𝐹𝐴 + 𝜆222𝐶𝑅𝑏− 𝐹ℎ

𝐶𝐺𝑊−𝐶𝑅

(1)

qGw Groundwater inflow (m3 day−1 m-2)

𝑄𝑅𝑥𝑤

𝑑𝐶𝑅𝑑𝑥

Change of 222Rn with distance along a stream receiving gw. inflow (Bq day−1 m-2)

𝐹𝐴 Flux of 222Rn to the atmosphere through the river surface boundary layer (Bq day−1 m-2)

𝜆222𝐶𝑅𝑏 Decay of 222Rn contained in the river volumen (Bq day−1 m-2)

𝐹ℎ 222Rn diffusive inflow from the hyporheic zone to the river volumen (Bq day−1 m-2)

𝐶𝐺𝑊 − 𝐶𝑅 Difference between conc. of 222Rn in inflowing groundwater & in river water (Bq m-3)

Cl mass balances (CMB) and water mass balances (RFMB) were performed for

discrete river volumes with equations (2) and (3) respectively. Assuming steady

state conditions:

V. METHODS

The effect of uncertainties on the estimated mass balances was quantified as

error propagation:

𝜎𝑄𝐺𝑊2 = 𝜎𝑋𝑖

𝜕𝑓

𝜕𝑋𝑖

2 𝑘

𝑖=1 (4)

𝑄𝐺𝑊 =𝑄𝑅𝐿·𝐶𝑙𝑅𝐿−𝑄𝑅0·𝐶𝑙𝑅0

𝐶𝑙𝐺𝑤−𝐶𝑙𝑅𝐿− 𝐸𝐶𝑙𝑅𝐿

(2)

(3) QGW = QRx + E − QR0 − P + possible lateral surface inputs

QGw is groundwater inflow (m3 day−1)

QRL is stream outflow (m3 day−1) from reach at x = L

ClRL is concentration of Cl− (mg L−1) in water at x = L

QRo is stream inflow (m3 day−1) in upstream reach x=0

ClRo is concentration of Cl− (mg L−1) in water at x=0

ClGw is concentration of Cl− (mg L−1) in groundwater

E is evaporation rate (m day−1)

QGw is groundwater inflow (m3 day−1); QRx is stream outflow (m3 day−1) from reach at x = x

QRo is stream inflow (m3 day−1) in upstream reach x=0; E is evaporation rate (m day−1)

P is the direct precipitation on the reach (m3 day−1)

CMB:

RFMB:

Ayna

Mundo River

Vadillos

Creek

N

0 20 km

VI. RESULTS. O1: Identification of groundwater

discharge areas

Observed increases in radon activity point to the occurrence of significant

groundwater discharge in four river tracts:

100-101

104-105

108-110

110-114

Liétor

Liétor

weir

Radon activity

0-150 (Bq·m-3)

150-450 (Bq·m-3)

450-750 (Bq·m-3)

750-1200 (Bq·m-3)

1200-1500 (Bq·m-3)

Very low

Low

Low-Medium

Medium

High

Classification (relative)

VII. RESULTS. O2: Quantification of groundwater

discharge flows by means of RMB

*Field conditions prevented to perform flow gauging at station 107

Groundwater discharge quantification in tracts 100-101 104-105 108-110 110-114

The main gaining reach was identified in Ayna, section 110-114.

Groundwater discharge estimated by RMB in the Ayna reach was ≈ 88 % total river flow measured (at the reach) in 2011, ≈ 65 % in 2012 and ≈ 66 % in 2013.

2011

2012

2013

Groundwater discharge estimates by RMB compared to values estimated by Cl mass balance (CMB) show a good similarity in tracts and/or dates where surface water inputs were null or very small.

Overall, total groundwater discharge estimated by RMB to the studied Mundo River length in July 2013 was ≈ 8-16 % of the total river flow measured by runoff mass balances.

VIII. RESULTS. O3: Checking the performance of RMB

QGW-RMB: Groundwater discharge after Rn mass balance

QGW-CMB: Groundwater discharge after Cl mass balance

Qt-RFMB: Total river flow by runoff mass balance

IX. CONCLUSIONS

Four significant areas of groundwater inflow to the Mundo River were identified, in which Ayna village area was the main gaining reach of the studied river length (66-88 % of the total groundwater discharge measured between 2011 and 2013).

Radon has proven to be a good tool to identify river reaches of diffuse groundwater discharge and also to quantify groundwater discharge during low water level conditions.

Estimated uncertainty (2σ) associated with QGW by RMB and CMB, and to Qt by RFMB added confidence to the results and conclusions.

This study is part of the work undertaken under the project REDESAC, funded by Spanish Ministry of Science and Innovation (MICCIN) CGL2009-2910-CO3

The first author is funded by Argentina's CONICET (National Scientific and Technical Research Council).

ACKNOWLEDGMENTS