River Tees - Water Quality Archive

The Environment Agency data portal allows access to water quality date from around the UK:

EA Water Quality Archive

A number of areas are sampled roughly monthly on the River Tees, you can find them all on the data portal.

Sampling points around the downstream Tees

Below are a number of key ones:

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How does this compare to other places in the country?

Make Tees data look very gappy? Why?

However,

seems to have the same gaps as Tees. Why?

But other data doesn't have the same gaps: