Methods and assumptions

This page describes the calculations the tool performs and the assumptions behind them. It is written for users who want to understand where the numbers come from — no familiarity with the legacy Excel tool or with hydrogeology software is needed.

The tool’s calculations are ported directly from the legacy Excel tool iMapBCDistDrawdown, developed by D. van Everdingen & M. Leahey (2024). Cell references throughout (e.g. Impact!Q2, Impact!U2) point to the exact formula in that workbook.

The Cooper-Jacob distance-drawdown formula

The tool estimates drawdown at any observation well using the Cooper-Jacob (1946) distance-drawdown approximation to the Theis (1935) well equation:

s(r, t) = (Q / 4πT) × ln(2.25 × T × t / (r² × S))

where

  • s — drawdown at the observation well (metres)
  • Q — pumping rate of the proposed well (m³/day)
  • T — aquifer transmissivity (m²/day)
  • S — aquifer storativity (dimensionless)
  • r — straight-line distance from the pumping well (metres)
  • t — duration of pumping (days)

The legacy Excel tool used the equivalent base-10 form

s = (2.303 × Q / 4πT) × log₁₀(2.25 × T × t / (S × r²))

The two forms produce identical numbers.

r → 0 fallback

If an observation point coincides with the pumping well (r = 0) the formula is undefined. The tool substitutes r = 0.1 m, matching the convention used in the legacy Excel tool (Impact!Q2), so the pumping well itself returns a finite, large drawdown rather than an error.

Cooper-Jacob validity check

Cooper-Jacob is an approximation that holds only when the dimensionless parameter

u = r² × S / (4 × T × t)

is small (u < 0.01). When u exceeds the threshold for a given distance and duration, the calculation is less reliable. The tool still reports the drawdown number but flags the affected wells in the per-well details table with a light-purple row tint — treat those values as advisory.

Default transmissivity and storativity (T, S)

Each aquifer in BCGW carries an aquifer subtype code (1a, 4b, 5b, and so on). The tool looks the subtype up in a table of default (T, S) values sourced from Wei et al. (2009) medians — the same table used by the legacy Excel tool (AquiferProperty_DB). The values can be overridden on the setup page if local data are available.

For karstic (subtype 5b) and unknown (subtype UNK) aquifers, no defaults are provided — the tool requires manual T and S entry.

Safe Available Drawdown (SAD)

For each observation well the tool computes the Safe Available Drawdown, a screening estimate of how much that well can withstand:

available drawdown = top − non-pumping water level + stickup

SAD = available drawdown × 0.7

where

  • top — the relevant top depth in metres. For unconfined sand-and- gravel wells this is the bottom of the well. For confined and fractured-bedrock wells it should be the top of the aquifer or the uppermost major water-bearing fracture, read from the driller’s log and entered via the per-well “Top of fracture / aquifer / screen” override.
  • non-pumping water level — the static water level reported by GWELLS.
  • stickup — the height of the well casing above the ground surface, if available.

The 0.7 (70%) safety factor matches the legacy Excel tool (Impact!U2).

Confined and fractured-bedrock wells

For confined or bedrock wells the unconfined formula above over-estimates SAD. The tool flags these wells with a “manual review of driller’s log recommended” note and exposes the per-well “Top of fracture / aquifer / screen” override so you can supply the correct top depth.

At-risk classification

Each observation well receives a status based on the ratio of predicted drawdown to SAD:

Status Meaning
OK Predicted impact is below 30% of SAD.
AT_RISK Predicted impact is 30% of SAD or more.
INSUFFICIENT_DATA SAD could not be computed (missing water level or well depth).
SUSPECT_DATA SAD was computed but is non-positive — the GWELLS baseline record is physically impossible and should be reviewed against the driller’s log.

The 30% threshold indicates that the anticipated drawdown impact is equal to 30% of the calculated Safe Available Drawdown. This threshold serves as a general screening guideline to help identify wells that may experience a substantial impact from the proposed groundwater diversion. Further interpretation and evaluation should be undertaken by, or in consultation with, a Regional Hydrogeologist or Qualified Professional.

The threshold itself matches the legacy Excel tool’s at-risk filter (Impact!V and the InputValues!B30 summary).

Reassigned aquifer material

GWELLS reports an aquifer material for many wells, but the legacy Excel tool computes a parallel “reassigned material” classification using the bedrock depth on the driller’s log (Impact!R):

  • If bedrock depth is recorded and (finished well depth − bedrock depth) is greater than 5 feet, classify the well as Bedrock.
  • Otherwise classify as Unconsolidated.
  • If neither is available, fall back to the GWELLS-reported material.

Both values appear in the per-well details table. The reassigned value is the one used for downstream interpretation.

Current version limitations (v1)

A couple of v1 simplifications worth keeping in mind when interpreting results. Both are user-interface limitations, not gaps in the underlying math.

Single pumping well (no superposition)

The v1 user interface accepts a single proposed pumping well. The underlying math already supports multiple pumping sources — their contributions are summed linearly, which is mathematically free for Cooper-Jacob (it is linear in Q). Multi-well (superposition) analyses are a future user-interface enhancement, not a new calculation. If you are screening a proposed development that will include several pumping wells, run the tool once for each, keep in mind the results do not account for interference between them, and use professional judgement to combine them.

Single (T, S) per aquifer subtype (no range)

Each aquifer subtype is assigned one default (T, S) pair drawn from the Wei et al. (2009) medians. Real aquifer properties vary within a subtype, sometimes by orders of magnitude — the median is a useful starting point but not the whole picture. The drawdown number the tool reports is therefore a point estimate using the median values, not a range. If you have local pumping-test or driller’s-log data for the specific aquifer, enter your own T and S via the Override default T / S toggle on the setup page to anchor the estimate to that site. A future version may report a range of drawdown estimates that reflects the within-subtype spread of T and S.

Assumptions and limitations

These are screening-level estimates. The tool assumes:

  • A confined or semi-confined aquifer of uniform transmissivity and storativity over the area of interest.
  • Continuous pumping at the chosen rate for the chosen duration.
  • No recharge during the pumping period — a worst-case dry-season assumption.
  • Straight-line distance to each observation well; no faults, barriers, or anisotropy are modelled.

The default 90-day pumping duration matches the dry-season convention used by the legacy Excel tool and is appropriate for screening across BC.

Always review. Results are advisory and must be reviewed by a qualified hydrogeologist. The tool is not a replacement for professional assessment.

Guidance on using this tool

The following guidance appears throughout the tool and in full in the PDF export. It is collected here as the canonical statement.

  • This tool estimates the predicted cone of depression resulting from pumping a well. It uses an analytical solution to provide a simplified representation of a complex natural system and may not be applicable in all situations. The results should not be relied upon as the sole basis for decision-making and do not replace the advice of a Qualified Professional or Regional Hydrogeologist.
  • Default aquifer parameter values for British Columbia are based on a limited dataset. Best practice is to evaluate a range of values and conduct a sensitivity analysis of the storage coefficient (S) and transmissivity (T) input parameters.
  • This tool utilizes a single analytical approach (Cooper-Jacob Method). Given the inherent uncertainties associated with aquifer properties, users are encouraged to conduct sensitivity analyses by running multiple simulations using a range of plausible aquifer parameter values to better understand the influence of those uncertainties on predicted drawdown.
  • Users are encouraged to verify information wherever possible, including reviewing scanned driller’s logs and other supporting documentation available through the associated GWELLS records.
  • If you have questions regarding the use of this tool or the interpretation of its results, please contact a Regional Hydrogeologist or Qualified Professional.

Running a sensitivity analysis today

The tool takes one T and one S per run, so a sensitivity analysis is currently a manual exercise: run the analysis several times with the Override default T / S toggle set to a range of plausible values, and compare the resulting impact percentages. Exporting each run to PDF gives you a set of comparable artifacts — each carries its own run ID and timestamp.

References

  • Cooper, H.H., and C.E. Jacob (1946). A generalized graphical method for evaluating formation constants and summarizing well-field history. Transactions, American Geophysical Union, 27(4), 526–534.
  • Theis, C.V. (1935). The relation between the lowering of the piezometric surface and the rate and duration of discharge of a well using ground-water storage. Transactions, American Geophysical Union, 16(2), 519–524.
  • Wei, M., Allen, D.M., Kohut, A.P., Grasby, S., Ronneseth, K., and Turner, B. (2009). Understanding the types of aquifers in the Canadian Cordillera hydrogeologic region to better manage and protect groundwater. Streamline Watershed Management Bulletin, 13(1), 10–18.
  • Legacy Excel tool: iMapBCDistDrawdown, developed by D. van Everdingen & M. Leahey, 2024. This tool’s screening calculations — the Cooper-Jacob form, the SAD formula, the reassigned-material rule, the chart layout, the unit list, the default duration, and the 30% at-risk threshold — are ported directly from that workbook.