After heavy rainfall, a well's test results may change while a nearby borehole's remain similar. This does not mean one type of source is always safe and another always unsuitable. Local conditions, structure, water movement, sampling time and even the point inside the home influence the difference. Comparing results requires a clear question and consistent procedure, not two numbers removed from their context.
Assess safety first
If the source or surroundings flooded, or there is a specific suspicion of contamination, arrange a safe alternative before waiting for a comparison table. Agree use with responsible specialists. Waiting for analysis must not mean water of uncertain condition can be used for drinking or food preparation simply because it was suitable before. Clarity and normal odour do not confirm every parameter.
The NVSPL notice on heavy rain and water quality discusses links between weather events and water risks. It provides a basis for targeted checks, not an automatic diagnosis for a particular plot. One downpour does not establish in advance which substance entered your source or its concentration.
Why may sources respond differently?
Their depths, aquifers, construction and possible surface entry points differ. Even two sources of the same type may have different conditions. A neighbour's test therefore cannot count as yours. A shared environmental event does not mean identical results everywhere or at the same time.
Water travel time matters too. A result immediately after rain may differ from a later sample, but that difference alone does not establish the cause. Agree repeat sampling timing and scope with specialists for the situation. Submitting random samples daily yourself may cost a great deal without providing a clear basis for comparison.
Describe the event properly
Record the heavy rain date, when the water change was noticed and whether direct flooding occurred. Note possible surrounding events: excavation, a wastewater system fault or a damaged cover. Photographs can help if taken safely. They must not replace testing, but convey details that may no longer be visible when the plot dries.
Distinguish observations from assumptions. 'Water stood around the well lining' is an observation. 'The neighbour's wastewater entered the well' is already a claim about cause that may lack evidence. A justified suspicion can warrant safety measures, but accusations and final technical conclusions must remain separate from initial information.
Comparison needs the same location
A sample at the source and one after filters do not represent the same thing. To assess a source change, choose the location and sampling method for that purpose. To check water being consumed, the whole system to the tap matters. State these objectives to the laboratory before obtaining containers and planning collection.
For example, an earlier sample may have come from the kitchen after a drinking water unit, while the new one came from an outdoor tap upstream of all filters. A difference between reports then does not prove rainfall alone caused it. First check sampling history, the schematic and work performed between samples. Only then is discussion of another cause meaningful.
Parameter programmes must be comparable
If one test measured only iron and another nitrate and microbiology, you cannot simply say 'the second water is worse'. The programmes answer different questions. Discuss with the laboratory what the event requires checking and which previous result you want to compare. Additional risks may need a parameter absent from the old programme.
CDC well-testing guidance emphasises checks after significant environmental changes. This general principle must not become an identical sampling schedule for everyone in Lithuania. Set the specific programme and timing for local conditions and the chosen laboratory, clearly distinguishing routine monitoring from testing after an event.
Units and laboratory notations
Compare complete parameter names and units, not just the larger number. Milligrams and micrograms per litre differ a thousandfold. Preserve less-than values and laboratory notes too. Do not arbitrarily convert them to zero merely to make percentage differences easier to calculate in a table.
If methods differed or the detection limit is unclear, contact the laboratory. Not every small numerical change means a significant deterioration in the source. Equally, an unfavourable result cannot be dismissed by saying 'perhaps the laboratory made a mistake'. Resolve uncertainty through professionally planned clarification, not by choosing the more convenient explanation.
What if the change is confirmed?
Investigate the possible entry route and an appropriate longer-term solution. Assessment of the structure, service entries or surroundings, cleaning, other professional work and verification may be needed. A new mechanical cartridge alone must not be treated as removal of all contamination. Installing a larger pump does not eliminate a water safety problem either.
Agree the repair and testing sequence in writing. Know what each stage will change and how the result will be assessed. Owners should not descend into wells or dose chemicals themselves. If several contractors are involved, clarify scopes and communication so the final report connects to the actual work history.
A brief example involving two samples
Suppose the sample before heavy rain was collected after a new filter, while the later one came from an old outdoor tap with a hose. Even very different results cannot isolate the rainfall's effect. First align the testing question and sampling location. If a difference remains after coordinated checks, it becomes more useful evidence for further assessment.
This example does not mean an unfavourable result can be ignored. Safety action and investigation run in parallel: first arrange appropriate use, then clarify which system part the sample represented and how a possible change can be reliably confirmed.
How can you retain a useful comparison?
Keep dates, sampling locations, programmes, units and system events in one table. Store original reports separately without rewriting them. The water safety guide explains the general importance of checks, while in an enquiry you can submit the technical context. Correct comparison helps select the next action, but must not become unjustified promotion or rejection of an entire source type.
Information sources
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