Assuming steady flow, the change in concentration C between points 1 and 2 along a pipe is:

```
C2 = C1 exp(-K*t)
```

where K is the overall rate coefficient and t is the travel time between 1 and 2 equal to:

```
t = L / v = L * (PI * D^2) / 4 / Q
```

where L is distance between 1 and 2, v is flow velocity, D is diameter and Q is flow rate. Since you didn’t specify a flow rate, let’s assume it produces a velocity of 1 m/sec. Therefore t is:

```
t = 2186 / 1 / 86400 = 0.025 days
```

so that K is:

```
K = ln(0.67/0.79) / 0.025 = -6.6 / day
```

K consists of both the bulk and wall reaction coefficients as follows:

```
K = Kb + (4/D)*Kw
```

where we assume that mass transfer is not rate-limiting. Since we know K, Kb and D we can solve for Kw:

```
Kw = (K - Kb) * (D/4) = (-6.6 - (-1)) * (0.5/4) = -0.7 m/day
```

Of course this all assumes a velocity of 1 m/s. To estimate the actual wall coefficient you must know the flow rate through the pipe when you took your concentration samples.