Question: Arsenic in groundwater. Refer to the Environmental Sci­ence & Technology (Jan. 2005) study of the reliability of a commercial kit to test for arsenic in groundwater, Exercise 11.20 (p. 526). Recall that you fit a first-order model for arsenic level (y) as a function of latitude, longitude, and depth to the data saved in the ASWELLS file. Conduct a complete residual analysis of the model. Do you recom­mend any model modifications?

(Data for first and last five wells shown)

Well	Latitude	Longitude	Depth	Arsenic
10	23.7887	90.6522	60	331
14	23.7886	90.6523	45	302
30	23.78	90.6517	45	193
59	23.7893	90.6525	125	232
85	23.7920	90.6140	150	19
7353	23.7949	90.6515	40	48
7357	23.7955	90.6515	30	172
7890	23.7658.	90.6312	60	175
7893	23.7656	90.6315	45	624
7970	23.7644	90.6303	30	254