Natural background levels assessment in groundwater
Ariane BLUM (BRGM, Water Division)
Frank WENDLAND, Ralf KUNKEL (Research Center Jülich, FZJ)
BRIDGE Final meeting – Paris, 15th December 2006 >2
Outlines of the presentation
> Introduction (definition and concepts)
> Overview of existing approaches to identify Natural Background Levels (NBL)
> Approach to derive NBLs
>2
Solution contents of groundwater depend on…
Natural influences
•
Groundwater covering layers•
Petrographical aquifer properties•
Hydrodynamical aquifer properties•
Hydrology•
RainfallsHuman impact
•
Landcover (changes)•
Water regulations•
Mining•
Point source pollution•
Diffuse intakes from agriculture and atmosphereNBL = “The concentration of a given element, species or chemical substance present in solution which is derived by natural processes from geological, biological or atmospheric sources”.
A strictly “natural” groundwater may be found at best in regionally
restricted areas, with mostly minor importance for water supply
BRIDGE Final meeting – Paris, 15th December 2006 >4
Quality versus status : general relationships
European Aquifer Typologies for referencing NBLs
Further criteria
-hydrodynamics (recharge, residence time, topography, leakage…)
- redox conditions
- particularities (Organic matter, dykes…)
- geological age
Prime criteria -
Petrography-
SalinityNBLs are different from one typology another
BRIDGE Final meeting – Paris, 15th December 2006 >6
The BRIDGE proposal to derive threshold values
?
Overview of existing approaches to identify NBL
Local scale approach (aquifer)
National scale approach (typology)
BRIDGE Final meeting – Paris, 15th December 2006 >8
Overview of existing approaches to identify NBL
• Evaluation of groundwater samples free from human impact
Only samples from deeper aquifers or from natural reservoirs can be used
• Hydrochemical simulation of solution processes in aquifers
Requires “deeper knowledge” about hydrochemistry, only works for well monitored small areas and is time-consuming
influenced component natural component sum of calculated natural and influenced components
observed concentrations (classified)
concentration
frequency
•
Evaluation of groundwater samples by separation methods1. Component separation by concentration distribution analysis Method requires “deeper knowledge” about statistical analysis
2. Preselection methods: Exclusion of samples with indicator substances for anthropogenic inputs above a certain value
Prerequisites for NBL derivation
Consequence for NBL derivation:
–
NBLs from former regional / national studies exist already and / or–
“Own” national or scientific approaches to derive NBLs exist already ... and can be taken into account for NBL derivation. Additionally…–
a general applicable, simple approach to derive NBLs is suggested by the BRIDGE project and–
reference values of NBLs are compiled by the BRIDGE project for the aquifer typologies (to be used in case there is a lack of data)•
The level of knowledge about hydrogeochemical transfers and processes varies a lot from one region to another.•
Approaches to assess NBLs are very different from country to country (in case they exist). There is no common approach in the EU to derive NBLs; many of the existing approaches date from before the WFD.•
Availability of groundwater quality data is very different.level of knowledge; available groundwater data
BRIDGE Final meeting – Paris, 15th December 2006 >10
Proposed simple approach to derive NBLs
NBL to be derived based on a simplified
(preselection) approach
Calculation of percentile 90 after a selection of the
samples
Groundwater Body
Which aquifer typology?
Confined or unconfined?
High level of knowledge/data
Well monitored GWB: many sampling sites, numerous chemical analysis geochemical
transfers and processes well known,NBLderived already….
NBL to be derived based on “own”
approach
Possibility to use percentile 97.7 of uninfluenced samples
Medium level of knowledge / some data
available
Some data are available but they are not sufficient to precisely define the geochemical
characteristics of the GWB
Low level of knowledge/
no data
No data available in the GWB (or just a few that are not sufficient to give the general chemical composition of the
GWB)
NBL to be derived based on WP2 statistical results
Use percentile 90 from statistical calculations made
in other countries for the same type of aquifer typology
Establish NBL
Pre-selection method to apply on a GWB
1. Identification of relevant parameters to exclude samples influenced by human activities
Purely anthropogenic substances (pesticides, COV...) Nitrate
Other parameters : K, Cl, Na, NH4+ (if no high NBL known!)
2. Exclusion of groundwater samples from the further assessment of NBLs in case
Nitrate > 10 mg/l (oxidized groundwaters)
ÆWarning: for anaerobic aquifers, a denitrification process can lead to a NO3concentration < 10 mg/l despite a human influence! Æuse other parameters. e.g. SO4, N2O, datation (CFC, T)
3
.NBLs are derived for the remaining data-base using e.g. simple
standard statistical methods and assessed as the concentration at the 90-percentile
If high level of knowledge and no human influence: p97.7 can be used
BRIDGE Final meeting – Paris, 15th December 2006 >12
Minimum requirements for groundwater data used for NBL derivation
• Samples with incorrect ion balance (exceeding 10%) should be removed
• Monitoring data not attachable to aquifer typologies should be removed
• Data from hydrothermal aquifers should be removed
• Data from salty aquifers should be removed (NaCl content of more than 1000mg/L; coastal or influenced by evaporates)
Æ
Salty aquifers must be considered as a separate type of aquifer and NBL should be, in this case derived with a case by case approachMinimum requirements for groundwater data used for NBL derivation
• Data from aerobic aquifers should be separated from those of anaerobic aquifers
(≥ 1 mg O2/l) and anaerobic (< 1 mg O2/l) aquifer conditions
(Note: Fe < 0.2, Mn < 0.05 mg/l (aerobic) or (Fe ≥ 0.2, Mn ≥ 0.5 mg/l) (anaerobic) condition may be used if no O2-data are available)
• All the data available can be used (no restriction on the time series to consider) but time series should be eliminated by median
averaging (in order to guarantee that all sampling sites contribute equally to the NBL derivation)
• For traces elements, limits quantification cannot be too high. In
particular, LQ equal to DWS should be removed
BRIDGE Final meeting – Paris, 15th December 2006 >14
